cifs.upcall: remove getpwuid() dependency

cifs.upcall: remove getpwuid() dependency

This patch removes getpwuid() and resort to getting UID/GID from
   /proc/pid/status

Changes:
- add get_uidgid() helper
- drop supplementary groups (i.e. move setgroups(0, NULL)) before even
  decoding spnego key -- those are never needed in any case

Signed-off-by: Enzo Matsumiya <ematsumiya@suse.de>
Reported-by: Shaomin Chen <eeesssooo020@gmail.com>
Acked-by: Paulo Alcantara (Red Hat) <pc@manguebit.org>
Acked-by: Pavel Shilovskiy <pshilovskiy@microsoft.com>
Reviewed-by: Shyam Prasad N <sprasad@microsoft.com>
Signed-off-by: Steve French <stfrench@microsoft.com>