complete_signal: kill always-true "core_state || !SIGNAL_GROUP_EXIT" check

The "(signal->core_state || !(signal->flags & SIGNAL_GROUP_EXIT))" check
in complete_signal() is not obvious at all, and in fact it only adds
unnecessary confusion: this condition is always true.

prepare_signal() does:

if (signal->flags & SIGNAL_GROUP_EXIT) {
if (signal->core_state)
return sig == SIGKILL;
/*
* The process is in the middle of dying, drop the signal.
*/
return false;
}

This means that "!signal->core_state && (signal->flags &
SIGNAL_GROUP_EXIT)" in complete_signal() is never possible.

If SIGNAL_GROUP_EXIT is set, prepare_signal() can only return true if
signal->core_state is not NULL.

Link: https://lkml.kernel.org/r/aZsfkDhnqJ4s1oTs@redhat.com
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Kees Cook <kees@kernel.org>
Cc: Mateusz Guzik <mjguzik@gmail.com>
Cc; Deepanshu Kartikey <kartikey406@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>