scm: Capture the full credentials of the scm sender

This patch corrects an erroneous update of credential's gid with uid
introduced in commit 257b5358 since 2.6.36.

Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
Acked-by: Eric Dumazet <eric.dumazet@gmail.com>
Reviewed-by: James Morris <jmorris@namei.org>
Signed-off-by: David S. Miller <davem@davemloft.net>