bcachefs: Check for unlinked, non-empty dirs in check_inode()

bcachefs: Check for unlinked, non-empty dirs in check_inode()

We want to check for this early so it can be reattached if necessary in
check_unreachable_inodes(); better than letting it be deleted and having
the children reattached, losing their filenames.

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