mm/oom_kill.c: introduce is_sysrq_oom helper

Introduce is_sysrq_oom helper function indicating oom kill triggered
by sysrq to improve readability.

No functional changes.

Signed-off-by: Yaowei Bai <bywxiaobai@163.com>
Acked-by: David Rientjes <rientjes@google.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>