bcachefs: Check inodes at start of fsck

This splits out checking inode nlinks from the rest of the inode checks
and moves most of the inode checks to the start of fsck, so that other
fsck passes can depend on it.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>