Fix unbalanced mutex in dma_pool_create().

dma_pool_create() needs to unlock the mutex in error case.  The bug was
introduced in the 3.16 by commit cc6b664aa26d ("mm/dmapool.c: remove
redundant NULL check for dev in dma_pool_create()")/

Signed-off-by: Krzysztof Hałasa <khc@piap.pl>
Cc: stable@vger.kernel.org  # v3.16
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/dmapool.c

@@ -176,7 +176,7 @@ struct dma_pool *dma_pool_create(const char *name, struct device *dev,
 	if (list_empty(&dev->dma_pools) &&
 	    device_create_file(dev, &dev_attr_pools)) {
 		kfree(retval);
-		return NULL;
+		retval = NULL;
 	} else
 		list_add(&retval->pools, &dev->dma_pools);
 	mutex_unlock(&pools_lock);