powerpc/spufs: convert userns uid/gid mount options to kuid/kgid

Acked-by: Jeremy Kerr <jk@ozlabs.org>
Tested-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: Dwight Engen <dwight.engen@oracle.com>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Ben Myers <bpm@sgi.com>