bcachefs: fsck: Fix check_directory_structure when no check_dirents

bcachefs: fsck: Fix check_directory_structure when no check_dirents

check_directory_structure runs after check_dirents, so it expects that
it won't see any inodes with missing backpointers - normally.

But online fsck can't run check_dirents yet, or the user might only be
running a specific pass, so we need to be careful that this isn't an
error. If an inode is unreachable, that's handled by a separate pass.

Also, add a new 'bch2_inode_has_backpointer()' helper, since we were
doing this inconsistently.

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