fuse2fs: fix race condition in op_destroy

fuse2fs: fix race condition in op_destroy

On a regular fuse server (i.e. one not running in fuseblk mode), libfuse
synthesizes and dispatches a FUSE_DESTROY command as soon as the event
dispatch loop terminates after the kernel disconnects /dev/fuse.
Unfortunately, this is done without coordinating with any other threads
that may have already received a real FUSE command from the kernel.

In other words, FUSE_DESTROY can run in parallel with other
fuse_operations.  Therefore, we must guard the body of this function
with the BKL just like any other fuse operation or risk races within
libext2fs.  If we're lucky, we trash the ext2_filsys state and
generic/488 will crash.

[23512.452451] [U] fuse: reading device: Software caused connection abort
[23512.453886] [U] fuse: reading device: Software caused connection abort

If we're not lucky, it corrupts the ondisk filesystem resulting in a
e2fsck complaining as well.

Cc: linux-ext4@vger.kernel.org # v1.43
Fixes: 81cbf1ef4f5dab ("misc: add fuse2fs, a FUSE server for e2fsprogs")
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Link: https://lore.kernel.org/r/20250709165152.GE2672022@frogsfrogsfrogs
Signed-off-by: Theodore Ts'o <tytso@mit.edu>