selftests/bpf: Fix warning comparing pointer to 0

Fix the following coccicheck warnings:

  ./tools/testing/selftests/bpf/progs/fexit_test.c:77:15-16: WARNING
  comparing pointer to 0.

  ./tools/testing/selftests/bpf/progs/fexit_test.c:68:12-13: WARNING
  comparing pointer to 0.
Reported-by: NAbaci Robot <abaci@linux.alibaba.com>
Signed-off-by: NJiapeng Chong <jiapeng.chong@linux.alibaba.com>
Signed-off-by: NDaniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/1615881577-3493-1-git-send-email-jiapeng.chong@linux.alibaba.com

tools/testing/selftests/bpf/progs/fexit_test.c

@@ -65,7 +65,7 @@ __u64 test7_result = 0;
 SEC("fexit/bpf_fentry_test7")
 int BPF_PROG(test7, struct bpf_fentry_test_t *arg)
 {
-	if (arg == 0)
+	if (!arg)
 		test7_result = 1;
 	return 0;
 }
@@ -74,7 +74,7 @@ __u64 test8_result = 0;
 SEC("fexit/bpf_fentry_test8")
 int BPF_PROG(test8, struct bpf_fentry_test_t *arg)
 {
-	if (arg->a == 0)
+	if (!arg->a)
 		test8_result = 1;
 	return 0;
 }