bcachefs: Only check inode i_nlink during full fsck

bcachefs: Only check inode i_nlink during full fsck

Now that all filesystem operatinos that manipulate the filesystem
heirachy and i_nlink are fully atomic, we can add a feature bit to
indicate i_nlink doesn't need to be checked.

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