bpf: Fix warning comparing pointer to 0

Fix the following coccicheck warning:

./tools/testing/selftests/bpf/progs/fentry_test.c:67:12-13: WARNING
comparing pointer to 0.
Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/1615360714-30381-1-git-send-email-jiapeng.chong@linux.alibaba.com

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

@@ -64,7 +64,7 @@ __u64 test7_result = 0;
 SEC("fentry/bpf_fentry_test7")
 int BPF_PROG(test7, struct bpf_fentry_test_t *arg)
 {
-	if (arg == 0)
+	if (!arg)
 		test7_result = 1;
 	return 0;
 }