swiotlb: bail out of swiotlb_init_late() if swiotlb is already allocated

If swiotlb is allocated, immediately return 0, so callers do not have to
check io_tlb_default_mem.nslabs explicitly.
Signed-off-by: Petr Tesarik <petr.tesarik.ext@huawei.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>

arch/arm/xen/mm.c

@@ -125,12 +125,10 @@ static int __init xen_mm_init(void)
 		return 0;
 
 	/* we can work with the default swiotlb */
-	if (!io_tlb_default_mem.nslabs) {
 	rc = swiotlb_init_late(swiotlb_size_or_default(),
 			       xen_swiotlb_gfp(), NULL);
 	if (rc < 0)
 		return rc;
-	}
 
 	cflush.op = 0;
 	cflush.a.dev_bus_addr = 0;

kernel/dma/swiotlb.c

@@ -384,6 +384,9 @@ int swiotlb_init_late(size_t size, gfp_t gfp_mask,
 	bool retried = false;
 	int rc = 0;
 
+	if (io_tlb_default_mem.nslabs)
+		return 0;
+
 	if (swiotlb_force_disable)
 		return 0;