selftests/bpf: Fix endianness issue in test_sockopt_sk

getsetsockopt() calls getsockopt() with optlen == 1, but then checks
the resulting int. It is ok on little endian, but not on big endian.

Fix by checking char instead.

Fixes: 8a027dc0 ("selftests/bpf: add sockopt test that exercises sk helpers")
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/20200915113928.3768496-1-iii@linux.ibm.com

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

@@ -45,9 +45,9 @@ static int getsetsockopt(void)
 		goto err;
 	}
 
-	if (*(int *)big_buf != 0x08) {
+	if (*big_buf != 0x08) {
 		log_err("Unexpected getsockopt(IP_TOS) optval 0x%x != 0x08",
-			*(int *)big_buf);
+			(int)*big_buf);
 		goto err;
 	}