selftests/bpf: Make test_varlen work with 32-bit user-space arch

Despite bpftool generating data section memory layout that will work for
32-bit architectures on user-space side, BPF programs should be careful to not
use ambiguous types like `long`, which have different size in 32-bit and
64-bit environments. Fix that in test by using __u64 explicitly, which is
a recommended approach anyway.

Signed-off-by: Andrii Nakryiko <[e-mail]>
Signed-off-by: Alexei Starovoitov <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

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

···
 	CHECK_VAL(bss->payload1_len2, size2);
 	CHECK_VAL(bss->total1, size1 + size2);
 	CHECK(memcmp(bss->payload1, exp_str, size1 + size2), "content_check",
-	      "doesn't match!");
+	      "doesn't match!\n");
 
 	CHECK_VAL(data->payload2_len1, size1);
 	CHECK_VAL(data->payload2_len2, size2);
 	CHECK_VAL(data->total2, size1 + size2);
 	CHECK(memcmp(data->payload2, exp_str, size1 + size2), "content_check",
-	      "doesn't match!");
+	      "doesn't match!\n");
 
 	CHECK_VAL(data->payload3_len1, size1);
 	CHECK_VAL(data->payload3_len2, size2);
 	CHECK_VAL(data->total3, size1 + size2);
 	CHECK(memcmp(data->payload3, exp_str, size1 + size2), "content_check",
-	      "doesn't match!");
+	      "doesn't match!\n");
 
 	CHECK_VAL(data->payload4_len1, size1);
 	CHECK_VAL(data->payload4_len2, size2);
 	CHECK_VAL(data->total4, size1 + size2);
 	CHECK(memcmp(data->payload4, exp_str, size1 + size2), "content_check",
-	      "doesn't match!");
+	      "doesn't match!\n");
 cleanup:
 	test_varlen__destroy(skel);
 }

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

···
 bool capture = false;
 
 /* .bss */
-long payload1_len1 = 0;
-long payload1_len2 = 0;
-long total1 = 0;
+__u64 payload1_len1 = 0;
+__u64 payload1_len2 = 0;
+__u64 total1 = 0;
 char payload1[MAX_LEN + MAX_LEN] = {};
 
 /* .data */