btrfs: add forward declarations and headers, part 1

Do a cleanup in the short headers:

- add forward declarations for types referenced by pointers
- add includes when types need them

This fixes potential compilation problems if the headers are reordered
or the missing includes are not provided indirectly.

Signed-off-by: David Sterba <dsterba@suse.com>

btrfs: remove old mount API code

Now that we've switched to the new mount API, remove the old stuff.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Acked-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

btrfs: switch to the new mount API

Now that we have all of the parts in place to use the new mount API,
switch our fs_type to use the new callbacks.

There are a few things that have to be done at the same time because of
the order of operations changes that come along with the new mount API.
These must be done in the same patch otherwise things will go wrong.

1. Export and use btrfs_check_options in open_ctree(). This is because
the options are done ahead of time, and we need to check them once we
have the feature flags loaded.

2. Update the free space cache settings. Since we're coming in with the
options already set we need to make sure we don't undo what the user
has asked for.

3. Set our sb_flags at init_fs_context time, the fs_context stuff is
trying to manage the sb_flagss itself, so move that into
init_fs_context and out of the fill super part.

Additionally I've marked the unused functions with __maybe_unused and
will remove them in a future patch.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Acked-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

btrfs: move space cache settings into open_ctree

Currently we pre-load the space cache settings in btrfs_parse_options,
however when we switch to the new mount API the mount option parsing
will happen before we have the super block loaded. Add a helper to set
the appropriate options based on the fs settings, this will allow us to
have consistent free space cache settings.

This also folds in the space cache related decisions we make for subpage
sectorsize support, so all of this is done in one place.

Since this was being called by parse options it looks like we're
changing the behavior of remount, but in fact we aren't. The
pre-loading of the free space cache settings is done because we want to
handle the case of users not using any space_cache options, we'll derive
the appropriate mount option based on the on disk state. On remount
this wouldn't reset anything as we'll have cleared the v1 cache
generation if we mounted -o nospace_cache. Similarly it's impossible to
turn off the free space tree without specifically saying -o
nospace_cache,clear_cache, which will delete the free space tree and
clear the compat_ro option. Again in this case calling this code in
remount wouldn't result in any change.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Acked-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

btrfs: move super_block specific helpers into super.h

This will make syncing fs.h to user space a little easier if we can pull
the super block specific helpers out of fs.h and put them in super.h.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

btrfs: move super prototypes into super.h

Move these out of ctree.h into super.h to cut down on code in ctree.h.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>