async_tx: avoid the async xor_zero_sum path when src_cnt > device->max_xor

If the channel cannot perform the operation in one call to
->device_prep_dma_zero_sum, then fallback to the xor+page_is_zero path.
This only affects users with arrays larger than 16 devices on iop13xx or
32 devices on iop3xx.

Cc: <stable@kernel.org>
Cc: Neil Brown <neilb@suse.de>
Signed-off-by: NDan Williams <dan.j.williams@intel.com>

crypto/async_tx/async_xor.c

@@ -271,7 +271,7 @@ async_xor_zero_sum(struct page *dest, struct page **src_list,
 
 	BUG_ON(src_cnt <= 1);
 
-	if (device) {
+	if (device && src_cnt <= device->max_xor) {
 		dma_addr_t *dma_src = (dma_addr_t *) src_list;
 		unsigned long dma_prep_flags = cb_fn ? DMA_PREP_INTERRUPT : 0;
 		int i;