bcachefs: Check for unlinked inodes with dirents

link count works differently in bcachefs - it's only nonzero for files
with multiple hardlinks, which means we can also avoid checking it
except for files that are known to have hardlinks.

That means we need a few different checks instead; in particular, we
don't want fsck to delet a file that has a dirent pointing to it.

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