signal: Ensure SIGNAL_GROUP_EXIT gets set in do_group_exit

signal: Ensure SIGNAL_GROUP_EXIT gets set in do_group_exit

The function do_group_exit has an optimization that avoids taking
siglock and doing the work to find other threads in the signal group
and shutting them down.

It is very desirable for SIGNAL_GROUP_EXIT to always been set whenever
it is decided for the process to exit.  That ensures only a single
place needs to be tested, and a single bit of state needs to be looked
at.  This makes the optimization in do_group_exit counter productive.

Make the code and maintenance simpler by removing this unnecessary
option.

Link: https://lkml.kernel.org/r/87letod4v3.fsf_-_@email.froward.int.ebiederm.org
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>