userns: On alpha modify linux_to_osf_stat to use convert from kuids and kgids

Silencing build errors and potentially allowing people to use osf
system calls in from processes running in a non-default user namespace.

It seems this stat call was missed in my first round of converting the
stat system calls, bother.

Cc: Richard Henderson <rth@twiddle.net>
Cc: Ivan Kokshaysky <ink@jurassic.park.msu.ru>
Cc: Matt Turner <mattst88@gmail.com>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>