xfs: publish UUID in struct super_block

Copy the uuid of the filesystem to struct super_block s_uuid field,
as several other filesystems already do.  Copy regardless of the nouuid
mount option, because other filesystems also do not guaranty uniqueness
of the s_uuid field in super_block struct.

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>