selftests/bpf: fix "ctx:write sysctl:write read ok" on s390

"ctx:write sysctl:write read ok" fails on s390 because it reads the
first byte of an int assuming it's the least-significant one, which
is not the case on big-endian arches. Since we are not testing narrow
accesses here (there is e.g. "ctx:file_pos sysctl:read read ok narrow"
for that), simply read the whole int.

Fixes: 1f5fa9ab ("selftests/bpf: Test BPF_CGROUP_SYSCTL")
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

tools/testing/selftests/bpf/test_sysctl.c

@@ -100,7 +100,7 @@ static struct sysctl_test tests[] = {
 		.descr = "ctx:write sysctl:write read ok",
 		.insns = {
 			/* If (write) */
-			BPF_LDX_MEM(BPF_B, BPF_REG_7, BPF_REG_1,
+			BPF_LDX_MEM(BPF_W, BPF_REG_7, BPF_REG_1,
 				    offsetof(struct bpf_sysctl, write)),
 			BPF_JMP_IMM(BPF_JNE, BPF_REG_7, 1, 2),