signal: During coredumps set SIGNAL_GROUP_EXIT in zap_process

There are only a few places that test SIGNAL_GROUP_EXIT and
are not also already testing SIGNAL_GROUP_COREDUMP.

This will not affect the callers of signal_group_exit as zap_process
also sets group_exit_task so signal_group_exit will continue to return
true at the same times.

This does not affect wait_task_zombie as the none of the threads
wind up in EXIT_ZOMBIE state during a coredump.

This does not affect oom_kill.c:__task_will_free_mem as
sig->core_state is tested and handled before SIGNAL_GROUP_EXIT is
tested for.

This does not affect complete_signal as signal->core_state is tested
for to ensure the coredump case is handled appropriately.

Link: https://lkml.kernel.org/r/20211213225350.27481-4-ebiederm@xmission.com


Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>