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: 358be656406d ("selftests/net: add txring_overwrite")
Signed-off-by: Frank de Brabander <[e-mail]>
Acked-by: Willem de Bruijn <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

tools/testing/selftests/net/txring_overwrite.c

···
 
 	*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;