btrfs: always wait for ordered extents to avoid OE races

In the Linux kernel, the following vulnerability has been resolved:

btrfs: always wait for ordered extents to avoid OE races

[BUG]
Syzbot reported a bug that there can be conflicting OEs for the same
range:

BTRFS critical (device loop4): panic in insert_ordered_extent:264: overlapping ordered extents, existing oe file_offset 16384 num_bytes 430080 flags 0x1089, new oe file_offset 16384 num_bytes 430080 flags 0x80 (errno=-17 Object alrea[ 179.162726][ T6897] BTRFS critical (device loop4): panic in insert_ordered_extent:264: overlapping ordered extents, existing oe file_offset 16384 num_bytes 430080 flags 0x1089, new oe file_offset 16384 num_bytes 430080 flags 0x80 (errno=-17 Object already exists)
------------[ cut here ]------------
kernel BUG at fs/btrfs/ordered-data.c:264!
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 05/09/2026
RIP: 0010:btrfs_alloc_ordered_extent+0x943/0xad0
Call Trace:
<TASK>
cow_file_range+0x744/0x12a0
fallback_to_cow+0x5ea/0xa00
run_delalloc_nocow+0x110c/0x17a0
btrfs_run_delalloc_range+0xbe4/0x1c20
writepage_delalloc+0x104d/0x1ba0
btrfs_writepages+0x1667/0x28b0
do_writepages+0x338/0x560
filemap_fdatawrite_range+0x1f2/0x300
btrfs_fdatawrite_range+0x54/0xf0
btrfs_direct_write+0x6a0/0xc30
btrfs_do_write_iter+0x329/0x790
do_iter_readv_writev+0x624/0x8d0
vfs_writev+0x34c/0x990
__se_sys_pwritev2+0x17a/0x2a0
do_syscall_64+0x174/0x580
entry_SYSCALL_64_after_hwframe+0x77/0x7f
</TASK>
---[ end trace 0000000000000000 ]---

[CAUSE]
Since commit ff66fe666233 ("btrfs: fix incorrect buffered IO fallback
for append direct writes"), if the direct IO finished short, we will
revert the isize back to the original one, so that append writes can be
respected during the buffered fallback.

Normally we rely on lock_and_cleanup_extent_if_need() function during
buffered writeback to wait for any existing ordered extents.

But that ordered extent waiting only happens if the start_pos is inside
the isize.
Since we have reverted the isize during failed direct IO, we will not
wait for any ordered extents.

This means we can have a race where the direct IO OE is still in the
tree, finished but not yet removed, then we're inserting the OE for the
buffered write, causing the above crash.

[FIX]
Make the OE wait to be unconditional, to handle the reverted isize
situation.

And since lock_and_cleanup_extent_if_need() now either lock the
extents or return -EAGAIN, also remove the branches that handles
no-extent-locked cases, and rename it to remove the "_if_need" suffix.

The following micro benchmark shows the runtime difference for
btrfs_buffered_write(), doing `xfs_io -f -c "pwrite 0 1m"` workload,
all values are the average runtime in nano seconds.

function runtime | before | after
-----------------------------------+-------------+---------------
lock_and_cleanup_extent_if_need() | 58.2 | 183.0
btrfs_buffered_write() | 2115.6 | 2973.3

The overall runtime of btrfs_buffered_write() is still pretty
tiny (still less than 3 micro seconds), I'd say the extra cost is still
acceptable.

An alternative to fix this problem is to wait ordered extents during
iomap_end() where the isize revert is done.

But that solution will break nowait requirement, as if a nowait direct
IO finished short, we have to wait for the OEs unconditionally or the
next append buffered IO can still hit the same problem.

So here we have to move the wait cost to buffered write, but at least
the code is slightly more streamline.