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: 6424e31b ("swiotlb: remove swiotlb_init_with_tbl and swiotlb_init_late_with_tbl")
Reported-by: Conor Dooley <Conor.Dooley@microchip.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Tested-by: Conor Dooley <Conor.Dooley@microchip.com>

kernel/dma/swiotlb.c

@@ -254,8 +254,10 @@ void __init swiotlb_init_remap(bool addressing_limit, unsigned int flags,
 		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));