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 <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200813204945.1020225-10-andriin@fb.com

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

@@ -15,9 +15,9 @@ int test_pid = 0;
 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 */