xfs: handle unallocated inodes in xfs_inode_from_disk

Handle inodes with a 0 di_mode in xfs_inode_from_disk, instead of partially
duplicating inode reading in xfs_iread.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>