userns: Convert s390 getting uid and gid system calls to use kuid and kgid

Convert getresuid, getresgid, getuid, geteuid, getgid, getegid

Convert struct cred kuids and kgids into userspace uids and gids when
returning them.

These s390 system calls slipped through the cracks in my first
round of converstions :(

Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>