tools/nolibc/string: Implement `strnlen()`

  size_t strnlen(const char *str, size_t maxlen);

The strnlen() function returns the number of bytes in the string
pointed to by sstr, excluding the terminating null byte ('\0'), but at
most maxlen. In doing this, strnlen() looks only at the first maxlen
characters in the string pointed to by str and never beyond str[maxlen-1].

The first use case of this function is for determining the memory
allocation size in the strndup() function.

Link: https://lore.kernel.org/lkml/CAOG64qMpEMh+EkOfjNdAoueC+uQyT2Uv3689_sOr37-JxdJf4g@mail.gmail.comSuggested-by: Alviro Iskandar Setiawan <alviro.iskandar@gnuweeb.org>
Acked-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Ammar Faizi <ammarfaizi2@gnuweeb.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

tools/include/nolibc/string.h

@@ -147,6 +147,15 @@ size_t nolibc_strlen(const char *str)
 #define strlen(str) nolibc_strlen((str))
 #endif
 
+static __attribute__((unused))
+size_t strnlen(const char *str, size_t maxlen)
+{
+	size_t len;
+
+	for (len = 0; (len < maxlen) && str[len]; len++);
+	return len;
+}
+
 static __attribute__((unused))
 size_t strlcat(char *dst, const char *src, size_t size)
 {