sched: numa: ksm: fix oops in task_numa_placment()

task_numa_placement() oopsed on NULL p->mm when task_numa_fault() got
called in the handling of break_ksm() for ksmd.  That might be a
peculiar case, which perhaps KSM could takes steps to avoid? but it's
more robust if task_numa_placement() allows for such a possibility.

Signed-off-by: Hugh Dickins <hughd@google.com>
Acked-by: Mel Gorman <mgorman@suse.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/sched/fair.c

@@ -793,8 +793,11 @@ unsigned int sysctl_numa_balancing_scan_delay = 1000;
 
 static void task_numa_placement(struct task_struct *p)
 {
-	int seq = ACCESS_ONCE(p->mm->numa_scan_seq);
+	int seq;
 
+	if (!p->mm)	/* for example, ksmd faulting in a user's mm */
+		return;
+	seq = ACCESS_ONCE(p->mm->numa_scan_seq);
 	if (p->numa_scan_seq == seq)
 		return;
 	p->numa_scan_seq = seq;