mm/oom_kill.c: avoid attempting to kill init sharing same memory

It's possible that an oom killed victim shares an ->mm with the init
process and thus oom_kill_process() would end up trying to kill init as
well.

This has been shown in practice:

	Out of memory: Kill process 9134 (init) score 3 or sacrifice child
	Killed process 9134 (init) total-vm:1868kB, anon-rss:84kB, file-rss:572kB
	Kill process 1 (init) sharing same memory
	...
	Kernel panic - not syncing: Attempted to kill init! exitcode=0x00000009

And this will result in a kernel panic.

If a process is forked by init and selected for oom kill while still
sharing init_mm, then it's likely this system is in a recoverable state.
However, it's better not to try to kill init and allow the machine to
panic due to unkillable processes.

[rientjes@google.com: rewrote changelog]
[akpm@linux-foundation.org: fix inverted test, per Ben]
Signed-off-by: Chen Jie <chenjie6@huawei.com>
Signed-off-by: David Rientjes <rientjes@google.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Acked-by: Hillf Danton <hillf.zj@alibaba-inc.com>
Cc: Ben Hutchings <ben@decadent.org.uk>
Cc: Li Zefan <lizefan@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>