selftests/bpf: Fix test for 4-byte load from remote_port on big-endian

The context access converter rewrites the 4-byte load from
bpf_sk_lookup->remote_port to a 2-byte load from bpf_sk_lookup_kern
structure.

It means that we cannot treat the destination register contents as a 32-bit
value, or the code will not be portable across big- and little-endian
architectures.

This is exactly the same case as with 4-byte loads from bpf_sock->dst_port
so follow the approach outlined in [1] and treat the register contents as a
16-bit value in the test.

[1]: https://lore.kernel.org/bpf/20220317113920.1068535-5-jakub@cloudflare.com/

Fixes: 2ed0dc5937 ("selftests/bpf: Cover 4-byte load from remote_port in bpf_sk_lookup")
Signed-off-by: Jakub Sitnicki <jakub@cloudflare.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Link: https://lore.kernel.org/bpf/20220319183356.233666-4-jakub@cloudflare.com

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

@@ -418,9 +418,15 @@ int ctx_narrow_access(struct bpf_sk_lookup *ctx)
 	if (LSW(ctx->remote_port, 0) != SRC_PORT)
 		return SK_DROP;
 
-	/* Load from remote_port field with zero padding (backward compatibility) */
+	/*
+	 * NOTE: 4-byte load from bpf_sk_lookup at remote_port offset
+	 * is quirky. It gets rewritten by the access converter to a
+	 * 2-byte load for backward compatibility. Treating the load
+	 * result as a be16 value makes the code portable across
+	 * little- and big-endian platforms.
+	 */
 	val_u32 = *(__u32 *)&ctx->remote_port;
-	if (val_u32 != bpf_htonl(bpf_ntohs(SRC_PORT) << 16))
+	if (val_u32 != SRC_PORT)
 		return SK_DROP;
 
 	/* Narrow loads from local_port field. Expect DST_PORT. */