selftests/bpf: Fix error checking on reading the tcp_fastopen sysctl

There is a typo in checking the "saved_tcp_fo" and instead
"saved_tcp_syncookie" is checked again.  This patch fixes it
and also breaks them into separate if statements such that
the test will abort asap.

Reported-by: David Binderman <dcb314@hotmail.com>
Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/20200211175910.3235321-1-kafai@fb.com

tools/testing/selftests/bpf/prog_tests/select_reuseport.c

@@ -822,8 +822,10 @@ void test_select_reuseport(void)
 		goto out;
 
 	saved_tcp_fo = read_int_sysctl(TCP_FO_SYSCTL);
+	if (saved_tcp_fo < 0)
+		goto out;
 	saved_tcp_syncookie = read_int_sysctl(TCP_SYNCOOKIE_SYSCTL);
-	if (saved_tcp_syncookie < 0 || saved_tcp_syncookie < 0)
+	if (saved_tcp_syncookie < 0)
 		goto out;
 
 	if (enable_fastopen())