xfs: convert kuid_t to/from uid_t for internal structures

Use uint32 from init_user_ns for xfs internal uid/gid
representation in xfs_icdinode, xfs_dqid_t.

Reviewed-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Gao feng <gaofeng@cn.fujitsu.com>
Signed-off-by: Dwight Engen <dwight.engen@oracle.com>
Signed-off-by: Ben Myers <bpm@sgi.com>