getrusage: add the "signal_struct *sig" local variable

[ Upstream commit c7ac8231

 ]

No functional changes, cleanup/preparation.

Link: https://lkml.kernel.org/r/20230909172554.GA20441@redhat.com
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Stable-dep-of: daa694e4

 ("getrusage: move thread_group_cputime_adjusted() outside of lock_task_sighand()")
Signed-off-by: Sasha Levin <sashal@kernel.org>