bcachefs: kill INODE_LOCK, use lock_two_nondirectories()

In an ideal world, we'd have a common helper that could be used for
sorting a list of inodes into the correct lock order, and then the same
lock ordering could be used for any type of inode lock, not just
i_rwsem.

But the lock ordering rules for i_rwsem are a bit complicated, so -
abandon that dream for now and do it the more standard way.

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