lib/string_kunit: extend benchmarks to strnlen() and chr searches

lib/string_kunit: extend benchmarks to strnlen() and chr searches

Extend the string benchmarking suite to include strnlen(), strchr(),
and strrchr().

For character search functions strchr() and strrchr(), the benchmark
targets the NUL character. This ensures the entire string is scanned,
providing a consistent measure of full-length processing efficiency
comparable to strlen().

Suggested-by: Andy Shevchenko <andy@kernel.org>
Suggested-by: Eric Biggers <ebiggers@kernel.org>
Signed-off-by: Feng Jiang <jiangfeng@kylinos.cn>
Acked-by: Andy Shevchenko <andy@kernel.org>
Reviewed-by: Kees Cook <kees@kernel.org>
Link: https://patch.msgid.link/20260130025018.172925-6-jiangfeng@kylinos.cn
Signed-off-by: Paul Walmsley <pjw@kernel.org>