bcachefs: Array bounds fixes

It's no longer legal to use a zero size array as a flexible array
member - this causes UBSAN to complain.

This patch switches our zero size arrays to normal flexible array
members when possible, and inserts casts in other places (e.g. where we
use the zero size array as a marker partway through an array).

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

bcachefs: Kill bch2_sort_repack_merge()

The main function of bch2_sort_repack_merge() was to call .key_normalize
on every key, which drops stale (cached) pointers - it hasn't actually
merged extents in quite some time.

But bch2_gc_gens() now works on individual keys - we used to gc old gens
by rewriting entire btree nodes. With that gone, there's no need for
internal btree code to be calling .key_normalize anymore.

Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>

bcachefs: Kill support for !BTREE_NODE_NEW_EXTENT_OVERWRITE()

bcachefs has been aggressively migrating filesystems and btree nodes to
the new format for quite some time - this shouldn't affect anyone
anymore, and lets us delete a _lot_ of code. Also, it frees up
KEY_TYPE_discard for a new whiteout key type for snapshots.

Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

bcachefs: Kill btree_node_iter_large

Long overdue cleanup - this converts btree_node_iter_large uses to
sort_iter.

Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

bcachefs: Refactor whiteouts compaction

The whiteout compaction path - as opposed to just dropping whiteouts -
is now only needed for extents, and soon will only be needed for extent
btree nodes in the old format.

Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

bcachefs: Make bkey types globally unique

this lets us get rid of a lot of extra switch statements - in a lot of
places we dispatch on the btree node type, and then the key type, so
this is a nice cleanup across a lot of code.

Also improve the on disk format versioning stuff.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

bcachefs: Split out bkey_sort.c

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>