selftests/bpf: Fix u8 narrow load checks for bpf_sk_lookup remote_port

In commit 9a69e2b3 ("bpf: Make remote_port field in struct
bpf_sk_lookup 16-bit wide") ->remote_port field changed from __u32 to
__be16.

However, narrow load tests which exercise 1-byte sized loads from
offsetof(struct bpf_sk_lookup, remote_port) were not adopted to reflect the
change.

As a result, on little-endian we continue testing loads from addresses:

 - (__u8 *)&ctx->remote_port + 3
 - (__u8 *)&ctx->remote_port + 4

which map to the zero padding following the remote_port field, and don't
break the tests because there is no observable change.

While on big-endian, we observe breakage because tests expect to see zeros
for values loaded from:

 - (__u8 *)&ctx->remote_port - 1
 - (__u8 *)&ctx->remote_port - 2

Above addresses map to ->remote_ip6 field, which precedes ->remote_port,
and are populated during the bpf_sk_lookup IPv6 tests.

Unsurprisingly, on s390x we observe:

  #136/38 sk_lookup/narrow access to ctx v4:OK
  #136/39 sk_lookup/narrow access to ctx v6:FAIL

Fix it by removing the checks for 1-byte loads from offsets outside of the
->remote_port field.

Fixes: 9a69e2b3 ("bpf: Make remote_port field in struct bpf_sk_lookup 16-bit wide")
Suggested-by: Ilya Leoshkevich <iii@linux.ibm.com>
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/20220319183356.233666-3-jakub@cloudflare.com

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

@@ -413,8 +413,7 @@ int ctx_narrow_access(struct bpf_sk_lookup *ctx)
 
 	/* Narrow loads from remote_port field. Expect SRC_PORT. */
 	if (LSB(ctx->remote_port, 0) != ((SRC_PORT >> 0) & 0xff) ||
-	    LSB(ctx->remote_port, 1) != ((SRC_PORT >> 8) & 0xff) ||
-	    LSB(ctx->remote_port, 2) != 0 || LSB(ctx->remote_port, 3) != 0)
+	    LSB(ctx->remote_port, 1) != ((SRC_PORT >> 8) & 0xff))
 		return SK_DROP;
 	if (LSW(ctx->remote_port, 0) != SRC_PORT)
 		return SK_DROP;