swiotlb: fix use after free on error handling path

Don't dereference "mem" after it has been freed.  Flip the
two kfree()s around to address this bug.

Fixes: 26ffb91fa5e0 ("swiotlb: split up the global swiotlb lock")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>

kernel/dma/swiotlb.c

@@ -979,8 +979,8 @@ static int rmem_swiotlb_device_init(struct reserved_mem *rmem,
 		mem->areas = kcalloc(nareas, sizeof(*mem->areas),
 				GFP_KERNEL);
 		if (!mem->areas) {
-			kfree(mem);
 			kfree(mem->slots);
+			kfree(mem);
 			return -ENOMEM;
 		}