selftests/bpf: Fix string read in strncmp benchmark

selftests/bpf: Fix string read in strncmp benchmark

The strncmp benchmark uses the bpf_strncmp helper and a hand-written
loop to compare two strings. The values of the strings are filled from
userspace. One of the strings is non-const (in .bss) while the other is
const (in .rodata) since that is the requirement of bpf_strncmp.

The problem is that in the hand-written loop, Clang optimizes the reads
from the const string to always return 0 which breaks the benchmark.

Use barrier_var to prevent the optimization.

The effect can be seen on the strncmp-no-helper variant.

Before this change:

    # ./bench strncmp-no-helper
    Setting up benchmark 'strncmp-no-helper'...
    Benchmark 'strncmp-no-helper' started.
    Iter   0 (112.309us): hits    0.000M/s (  0.000M/prod), drops    0.000M/s, total operations    0.000M/s
    Iter   1 (-23.238us): hits    0.000M/s (  0.000M/prod), drops    0.000M/s, total operations    0.000M/s
    Iter   2 ( 58.994us): hits    0.000M/s (  0.000M/prod), drops    0.000M/s, total operations    0.000M/s
    Iter   3 (-30.466us): hits    0.000M/s (  0.000M/prod), drops    0.000M/s, total operations    0.000M/s
    Iter   4 ( 29.996us): hits    0.000M/s (  0.000M/prod), drops    0.000M/s, total operations    0.000M/s
    Iter   5 ( 16.949us): hits    0.000M/s (  0.000M/prod), drops    0.000M/s, total operations    0.000M/s
    Iter   6 (-60.035us): hits    0.000M/s (  0.000M/prod), drops    0.000M/s, total operations    0.000M/s
    Summary: hits    0.000 ± 0.000M/s (  0.000M/prod), drops    0.000 ± 0.000M/s, total operations    0.000 ± 0.000M/s

After this change:

    # ./bench strncmp-no-helper
    Setting up benchmark 'strncmp-no-helper'...
    Benchmark 'strncmp-no-helper' started.
    Iter   0 ( 77.711us): hits    5.534M/s (  5.534M/prod), drops    0.000M/s, total operations    5.534M/s
    Iter   1 ( 11.215us): hits    6.006M/s (  6.006M/prod), drops    0.000M/s, total operations    6.006M/s
    Iter   2 (-14.253us): hits    5.931M/s (  5.931M/prod), drops    0.000M/s, total operations    5.931M/s
    Iter   3 ( 59.087us): hits    6.005M/s (  6.005M/prod), drops    0.000M/s, total operations    6.005M/s
    Iter   4 (-21.379us): hits    6.010M/s (  6.010M/prod), drops    0.000M/s, total operations    6.010M/s
    Iter   5 (-20.310us): hits    5.861M/s (  5.861M/prod), drops    0.000M/s, total operations    5.861M/s
    Iter   6 ( 53.937us): hits    6.004M/s (  6.004M/prod), drops    0.000M/s, total operations    6.004M/s
    Summary: hits    5.969 ± 0.061M/s (  5.969M/prod), drops    0.000 ± 0.000M/s, total operations    5.969 ± 0.061M/s

Fixes: 9c42652f8be3 ("selftests/bpf: Add benchmark for bpf_strncmp() helper")
Suggested-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Viktor Malik <vmalik@redhat.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Hou Tao <houtao1@huawei.com>
Link: https://lore.kernel.org/bpf/20250313122852.1365202-1-vmalik@redhat.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>