fs: Fix possible UAF in mark_buffer_write_io_error()

fs: Fix possible UAF in mark_buffer_write_io_error()

When filesystem is freeing inode it calls mmb_invalidate() which removes
bhs from inode's metadata bh tracking and clears b_mmb for them. However
if the inode is getting deleted, we don't bother with calling mmb_sync()
before and thus these buffers can be under IO and we can be racing with
IO completion handler calling mark_buffer_write_io_error(). This race
can lead to mark_buffer_write_io_error() either hitting NULL pointer
reference or trying to operate on already freed inode. Protect the
mapping handling with RCU to make sure mmb and inode aren't freed before
we are done with them.

Reported-by: Sashiko <sashiko-bot@kernel.org>
Signed-off-by: Jan Kara <jack@suse.cz>
Link: https://patch.msgid.link/20260727104923.3828017-22-jack@suse.cz
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>