lowmemorykiller: fix memory corruption

We only synchronize with the oom reaper if tsk_is_oom_victim which is
not true if the final __mmput is called from a different context than
the oom victim exit path.  This can trivially happen from context of any
task which has grabbed mm reference.

This was first reported in upstream. Reference commit can be found
at commit 065e76dd01e4 ("mm, oom_reaper: fix memory corruption")

Change-Id: I5556bf2546d9c7b90688bec1f7f67ee83a4a86ee
Signed-off-by: Charan Teja Reddy <charante@codeaurora.org>