staging: lowmemorykiller: remove bogus NULL check

The NULL checking here doesn't make sense, so it causes a static checker
warning.  It turns out that p->mm can't be NULL so the inconsistency is
harmless and we should just remove the check.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>