swiotlb: fail map correctly with failed io_tlb_default_mem

In the failure case of trying to use a buffer which we'd previously
failed to allocate, the "!mem" condition is no longer sufficient since
io_tlb_default_mem became static and assigned by default. Update the
condition to work as intended per the rest of that conversion.

Fixes: 463e862ac6 ("swiotlb: Convert io_default_tlb_mem to static allocation")
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>

kernel/dma/swiotlb.c

@@ -584,7 +584,7 @@ phys_addr_t swiotlb_tbl_map_single(struct device *dev, phys_addr_t orig_addr,
 	int index;
 	phys_addr_t tlb_addr;
 
-	if (!mem)
+	if (!mem || !mem->nslabs)
 		panic("Can not allocate SWIOTLB buffer earlier and can't now provide you with the DMA bounce buffer");
 
 	if (cc_platform_has(CC_ATTR_MEM_ENCRYPT))