x86_64: During VM oom condition, kill all threads in process group

x86_64: During VM oom condition, kill all threads in process group

During a VM oom condition, kill all threads in the process group.

We have had complaints where a threaded application is left in a bad state
after one of it's threads is killed when we hit a VM: out_of_memory condition.

Killing just one of the process threads can leave the application in a bad
state, whereas killing the entire process group would allow for the
application to restart, or otherwise handled, and makes it very obvious that
something has gone wrong.

This change allows the entire process group to be taken down, rather than just
the one thread.

Signed-off-by: Will <will_schmidt@vnet.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>