selftests: txring_overwrite: fix incorrect test of mmap() return value

If mmap() fails it returns MAP_FAILED, which is defined as ((void *) -1).
The current if-statement incorrectly tests if *ring is NULL.

Fixes: 358be656 ("selftests/net: add txring_overwrite")
Signed-off-by: NFrank de Brabander <debrabander@gmail.com>
Acked-by: NWillem de Bruijn <willemb@google.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

tools/testing/selftests/net/txring_overwrite.c

@@ -113,7 +113,7 @@ static int setup_tx(char **ring)
 
 	*ring = mmap(0, req.tp_block_size * req.tp_block_nr,
 		     PROT_READ | PROT_WRITE, MAP_SHARED, fdt, 0);
-	if (!*ring)
+	if (*ring == MAP_FAILED)
 		error(1, errno, "mmap");
 
 	return fdt;