Check recvmmsg exists before entering fuzzy loop

Avoid thread B entering infinite loop if recvmmsg doesn't exist causing
tst_brk to be called and thread A to make an ungraceful exit.

A more general fix can be added to tst_fuzzy_sync as well, but will take
longer to develop.

Signed-off-by: Richard Palethorpe <[email protected]>
Reported-by: Li Wang <[email protected]>
Reviewed-by: Li Wang <[email protected]>
Acked-by: Cyril Hrubis <[email protected]>

Author: Richard Palethorpe

testcases/cve/cve-2016-7117.c

@@ -99,6 +99,8 @@ static void setup(void)
 {
 	fzsync_pair.min_samples = 10000;
 
+	tst_syscall(__NR_recvmmsg, 0, 0, 0, 0, 0);
+
 	tst_fzsync_pair_init(&fzsync_pair);
 }