selftests/bpf: Mask bpf_csum_diff() return value to 16 bits in test_verifier

The verifier test labelled "valid read map access into a read-only array
2" calls the bpf_csum_diff() helper and checks its return value. However,
architecture implementations of csum_partial() (which is what the helper
uses) differ in whether they fold the return value to 16 bit or not. For
example, x86 version has ...

	if (unlikely(odd)) {
		result = from32to16(result);
		result = ((result >> 8) & 0xff) | ((result & 0xff) << 8);
	}

... while generic lib/checksum.c does:

	result = from32to16(result);
	if (odd)
		result = ((result >> 8) & 0xff) | ((result & 0xff) << 8);

This makes the helper return different values on different architectures,
breaking the test on non-x86. To fix this, add an additional instruction
to always mask the return value to 16 bits, and update the expected return
value accordingly.

Fixes: fb2abb73e5 ("bpf, selftest: test {rd, wr}only flags and direct value access")
Signed-off-by: Yauheni Kaliuta <yauheni.kaliuta@redhat.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20210228103017.320240-1-yauheni.kaliuta@redhat.com

tools/testing/selftests/bpf/verifier/array_access.c

@@ -250,12 +250,13 @@
 	BPF_MOV64_IMM(BPF_REG_5, 0),
 	BPF_RAW_INSN(BPF_JMP | BPF_CALL, 0, 0, 0,
 		     BPF_FUNC_csum_diff),
+	BPF_ALU64_IMM(BPF_AND, BPF_REG_0, 0xffff),
 	BPF_EXIT_INSN(),
 	},
 	.prog_type = BPF_PROG_TYPE_SCHED_CLS,
 	.fixup_map_array_ro = { 3 },
 	.result = ACCEPT,
-	.retval = -29,
+	.retval = 65507,
 },
 {
 	"invalid write map access into a read-only array 1",