selftests/net: psock_fanout seg faults in sock_fanout_read_ring()

The while loop in sock_fanout_read_ring() checks mmap region
bounds after access, causing it to segfault. Fix it to check
count before accessing header->tp_status. This problem can be
reproduced consistently when the test in run as follows:

    make -C tools/testing/selftests TARGETS=net run_tests
    or
    make run_tests from tools/testing/selftests
    or
    make run_test from tools/testing/selftests/net
Signed-off-by: NShuah Khan <shuahkh@osg.samsung.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

tools/testing/selftests/net/psock_fanout.c

@@ -128,7 +128,7 @@ static int sock_fanout_read_ring(int fd, void *ring)
 	struct tpacket2_hdr *header = ring;
 	int count = 0;
 
-	while (header->tp_status & TP_STATUS_USER && count < RING_NUM_FRAMES) {
+	while (count < RING_NUM_FRAMES && header->tp_status & TP_STATUS_USER) {
 		count++;
 		header = ring + (count * getpagesize());
 	}