i386: replace hard-coded constant with appropriate macro from kernel.h

 * If there is a limit on the length of a valid string, you may wish to

 * consider using strnlen_user() instead.

 */

#define strlen_user(str) strnlen_user(str, ~0UL >> 1)

#define strlen_user(str) strnlen_user(str, LONG_MAX)

long strnlen_user(const char __user *str, long n);

unsigned long __must_check clear_user(void __user *mem, unsigned long len);