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

In commit 9a69e2b385f4 ("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: 9a69e2b385f4 ("bpf: Make remote_port field in struct bpf_sk_lookup 16-bit wide")
Suggested-by: Ilya Leoshkevich <[email protected]>
Signed-off-by: Jakub Sitnicki <[email protected]>
Signed-off-by: Alexei Starovoitov <[email protected]>
Acked-by: Martin KaFai Lau <[email protected]>
Link: https://lore.kernel.org/bpf/[email protected]

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