btrfs: check status before the flags

Flags() uses fItem.
should help with #14486

Change-Id: I2593280f92e9843a2d57d66bf9b793748fb14f3d
Reviewed-on: https://review.haiku-os.org/c/haiku/+/5141
Reviewed-by: Adrien Destugues <pulkomandy@gmail.com>
Tested-by: Commit checker robot <no-reply+buildbot@haiku-os.org>

btrfs: do not create Journal and ExtentAllocator for read-only

They are not needed, and the ExtentAllocator seems to currently fail on
an empty volume I created for testing.

btrfs

* Fixed coding style for all files under /src/add-ons/kernel/file_systems/btrfs
* rewrote crc_table.cpp to generate CRCTable.cpp that conforms to coding style.

Change-Id: I90fcc143320c7a7e9a6dc174f0ad82f40bb8a9b6
Reviewed-on: https://review.haiku-os.org/c/1194
Reviewed-by: Jérôme Duval <jerome.duval@gmail.com>

style fixes

BTRFS: Add author and license.

Signed-off-by: Augustin Cavalier <waddlesplash@gmail.com>

BTRFS: Implement ExtentAllocator

There are 4 new classes, structs:
* CachedExtent, is a AVLTreeNode, caches the extent locating in extent tree, a extent can be free, allocated, metadata or a data extent. It also hold a references count,
that is incremented each time a new extent refer to it (COW) and item data, that is only for allocated extent (NULL for free).
* CachedTreeExtent, is a AVLTree, cache the whole extent tree and has CachedExtent as its node.
* BlockGroup represents the group of extents that represent the region for each type of allocated extent. For example, region for data extents, metada blocks. It
responsible for inserting nodes in CachedTreeExtent.
* And the final, ExtentAllocator it knows how to allocate/deallocate extents, but for now only the allocating is implemented, actually allocating and deallocating works
are already implemented, they are in functions _AddFreeExtent, _AddAllocatedExtent in CachedTreeExtent. However the deallocating is not needed for now, so it will be
finished later then.

Signed-off-by: Augustin Cavalier <waddlesplash@gmail.com>