swiotlb: don't panic when the swiotlb buffer can't be allocated

For historical reasons the switlb code paniced when the metadata could
not be allocated, but just printed a warning when the actual main
swiotlb buffer could not be allocated.  Restore this somewhat unexpected
behavior as changing it caused a boot failure on the Microchip RISC-V
PolarFire SoC Icicle kit.

Fixes: 6424e31b1c ("swiotlb: remove swiotlb_init_with_tbl and swiotlb_init_late_with_tbl")
Reported-by: Conor Dooley <[email protected]>
Signed-off-by: Christoph Hellwig <[email protected]>
Reviewed-by: Stefano Stabellini <[email protected]>
Acked-by: Conor Dooley <[email protected]>
Tested-by: Conor Dooley <[email protected]>

kernel/dma/swiotlb.c

@@ -254,8 +254,10 @@ retry:
 		tlb = memblock_alloc(bytes, PAGE_SIZE);
 	else
 		tlb = memblock_alloc_low(bytes, PAGE_SIZE);
-	if (!tlb)
-		panic("%s: failed to allocate tlb structure\n", __func__);
+	if (!tlb) {
+		pr_warn("%s: failed to allocate tlb structure\n", __func__);
+		return;
+	}
 
 	if (remap && remap(tlb, nslabs) < 0) {
 		memblock_free(tlb, PAGE_ALIGN(bytes));