seccomp: Don't lookup for pidns in restore_seccomp

Looking up for pid in nesting pidns supposed to be done
for non group leaders only, thus __export_restore_thread
do this check on its own and we don't have to make
a similar lookup especially on group leader where
pids in args never were valid.

Reported-by: Andrei Vagin <avagin@virtuozzo.com>
Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>