mm/slub.c: remove validation on cpu_slab in __flush_cpu_slab()

cpu_slab is a per cpu variable which is allocated in all or none.  If a
cpu_slab failed to be allocated, the slub is not usable.

We could use cpu_slab without validation in __flush_cpu_slab().

Link: http://lkml.kernel.org/r/20181103141218.22844-1-richard.weiyang@gmail.com
Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Christoph Lameter <cl@linux.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: David Rientjes <rientjes@google.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/slub.c

@@ -2317,12 +2317,10 @@ static inline void __flush_cpu_slab(struct kmem_cache *s, int cpu)
 {
 	struct kmem_cache_cpu *c = per_cpu_ptr(s->cpu_slab, cpu);
 
-	if (likely(c)) {
-		if (c->page)
-			flush_slab(s, c);
+	if (c->page)
+		flush_slab(s, c);
 
-		unfreeze_partials(s, c);
-	}
+	unfreeze_partials(s, c);
 }
 
 static void flush_cpu_slab(void *d)