percpu: resolve err may not be initialized in pcpu_alloc

From 4a42ecc735cff0015cc73c3d87edede631f4b885 Mon Sep 17 00:00:00 2001
From: Dennis Zhou <dennisz@fb.com>
Date: Wed, 21 Jun 2017 08:07:15 -0700

Add error message to out of space failure for atomic allocations in
percpu allocation path to fix -Wmaybe-uninitialized.
Signed-off-by: NDennis Zhou <dennisz@fb.com>
Reported-by: NStephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: NTejun Heo <tj@kernel.org>

mm/percpu.c

@@ -956,8 +956,10 @@ static void __percpu *pcpu_alloc(size_t size, size_t align, bool reserved,
 	 * tasks to create chunks simultaneously.  Serialize and create iff
 	 * there's still no empty chunk after grabbing the mutex.
 	 */
-	if (is_atomic)
+	if (is_atomic) {
+		err = "atomic alloc failed, no space left";
 		goto fail;
+	}
 
 	if (list_empty(&pcpu_slot[pcpu_nr_slots - 1])) {
 		chunk = pcpu_create_chunk();