exit: kill unnecessary thread_group_leader() checks in exit_notify() and do_notify_pa...

exit: kill unnecessary thread_group_leader() checks in exit_notify() and do_notify_parent()

thread_group_empty(tsk) is only possible if tsk is a group leader, and
thread_group_empty() already does the thread_group_leader() check.

So it makes no sense to check "thread_group_leader() &&
thread_group_empty()"; thread_group_empty() alone is enough.

Link: https://lkml.kernel.org/r/aZsfeegKZPZZszJh@redhat.com
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Mateusz Guzik <mjguzik@gmail.com>
Cc: Kees Cook <kees@kernel.org>
Cc; Deepanshu Kartikey <kartikey406@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>