dmaengine: dmatest: Add check for supported buffer count (sg_buffers)

When using dmatest with sg_buffers=128 I stumbled upon the problem, that
the "map_cnt" variable of "struct dmaengine_unmap_data" was set to 0.
"map_cnt" is an "u8" variable, resulting in an overrun when its
value is set to src_cnt + dst_cnt, to twice the sg_buffer value.

This patch adds a small check to dmatest, so that this confusing error
is detected and the test is aborted.
Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Kedareswara rao Appana <appanad@xilinx.com>
Cc: Vinod Koul <vinod.koul@intel.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>

drivers/dma/dmatest.c

@@ -535,6 +535,13 @@ static int dmatest_func(void *data)
 
 		total_tests++;
 
+		/* Check if buffer count fits into map count variable (u8) */
+		if ((src_cnt + dst_cnt) >= 255) {
+			pr_err("too many buffers (%d of 255 supported)\n",
+			       src_cnt + dst_cnt);
+			break;
+		}
+
 		if (1 << align > params->buf_size) {
 			pr_err("%u-byte buffer too small for %d-byte alignment\n",
 			       params->buf_size, 1 << align);