lib/string_kunit: add correctness test for strnlen()

lib/string_kunit: add correctness test for strnlen()

Add a KUnit test for strnlen() to verify correctness across
different string lengths and memory alignments. Use vmalloc()
to place the NUL character at the page boundary to ensure
over-reads are detected.

Suggested-by: Andy Shevchenko <andy@kernel.org>
Suggested-by: Kees Cook <kees@kernel.org>
Signed-off-by: Feng Jiang <jiangfeng@kylinos.cn>
Reviewed-by: Kees Cook <kees@kernel.org>
Link: https://patch.msgid.link/20260130025018.172925-3-jiangfeng@kylinos.cn
Signed-off-by: Paul Walmsley <pjw@kernel.org>