mm: simplify nodemask printing

alloc_warn() and dump_header() have to explicitly handle NULL nodemask
which forces both paths to use pr_cont.  We can do better.  printk
already handles NULL pointers properly so all we need is to teach
nodemask_pr_args to handle NULL nodemask carefully.  This allows
simplification of both alloc_warn() and dump_header() and gets rid of
pr_cont altogether.

This patch has been motivated by patch from Joe Perches

  http://lkml.kernel.org/r/b31236dfe3fc924054fd7842bde678e71d193638.1509991345.git.joe@perches.com

[akpm@linux-foundation.org: fix tile warning, per Arnd]
Link: http://lkml.kernel.org/r/20171109100531.3cn2hcqnuj7mjaju@dhcp22.suse.cz
Signed-off-by: Michal Hocko <mhocko@suse.com>
Acked-by: Joe Perches <joe@perches.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index a501a0a..c86fbd1 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -426,14 +426,10 @@ static void dump_tasks(struct mem_cgroup *memcg, const nodemask_t *nodemask)
 
 static void dump_header(struct oom_control *oc, struct task_struct *p)
 {
-	pr_warn("%s invoked oom-killer: gfp_mask=%#x(%pGg), nodemask=",
-		current->comm, oc->gfp_mask, &oc->gfp_mask);
-	if (oc->nodemask)
-		pr_cont("%*pbl", nodemask_pr_args(oc->nodemask));
-	else
-		pr_cont("(null)");
-	pr_cont(",  order=%d, oom_score_adj=%hd\n",
-		oc->order, current->signal->oom_score_adj);
+	pr_warn("%s invoked oom-killer: gfp_mask=%#x(%pGg), nodemask=%*pbl, order=%d, oom_score_adj=%hd\n",
+		current->comm, oc->gfp_mask, &oc->gfp_mask,
+		nodemask_pr_args(oc->nodemask), oc->order,
+			current->signal->oom_score_adj);
 	if (!IS_ENABLED(CONFIG_COMPACTION) && oc->order)
 		pr_warn("COMPACTION is disabled!!!\n");