signal: oom_kill_task: use SEND_SIG_FORCED instead of force_sig()

signal: oom_kill_task: use SEND_SIG_FORCED instead of force_sig()

Change oom_kill_task() to use do_send_sig_info(SEND_SIG_FORCED) instead
of force_sig(SIGKILL).  With the recent changes we do not need force_ to
kill the CLONE_NEWPID tasks.

And this is more correct.  force_sig() can race with the exiting thread
even if oom_kill_task() checks p->mm != NULL, while
do_send_sig_info(group => true) kille the whole process.

Signed-off-by: Oleg Nesterov <[email protected]>
Cc: Tejun Heo <[email protected]>
Cc: Anton Vorontsov <[email protected]>
Cc: "Eric W. Biederman" <[email protected]>
Cc: KOSAKI Motohiro <[email protected]>
Cc: David Rientjes <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>