xfs: dont set sb in xfs_mount_alloc()

When changing to use the new mount api the super block won't be
available when the xfs_mount struct is allocated so move setting the
super block in xfs_mount to xfs_fs_fill_super().

Signed-off-by: Ian Kent <raven@themaw.net>
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>