xfs: move v1 inode conversion to xfs_inode_from_disk

So we don't have to carry an di_onlink variable around anymore, move
the inode conversion from v1 inode format to v2 inode format into
xfs_inode_from_disk(). This means we can remove the di_onlink fields
from the struct xfs_icdinode.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Dave Chinner <david@fromorbit.com>