Drivers: hv : vmbus: Adding NULL pointer check

This patch fixes the following Coccinelle warning:
drivers/hv/ring_buffer.c:223: alloc with no test
Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: Lv Ruyi <lv.ruyi@zte.com.cn>
Link: https://lore.kernel.org/r/20211028104138.14576-1-lv.ruyi@zte.com.cnSigned-off-by: Wei Liu <wei.liu@kernel.org>

drivers/hv/ring_buffer.c

@@ -223,6 +223,8 @@ int hv_ringbuffer_init(struct hv_ring_buffer_info *ring_info,
 		pages_wraparound = kcalloc(page_cnt * 2 - 1,
 					   sizeof(struct page *),
 					   GFP_KERNEL);
+		if (!pages_wraparound)
+			return -ENOMEM;
 
 		pages_wraparound[0] = pages;
 		for (i = 0; i < 2 * (page_cnt - 1); i++)