asm-generic headers: add arch-specific __strnlen_user calling in uaccess.h

This patch changes the implementation of strnlen_user in include/asm-generic/uaccess.h.
Originally, it calls strlen() function directly, which may not correctly handle the access of
user space in most mmu-enabled architectures.
New __strnlen_user is added for using as an architecture specific function.

Signed-off-by: Guan Xuetao <[e-mail]>
Reviewed-by: Arnd Bergmann <[e-mail]>

include/asm-generic/uaccess.h

···
  *
  * Return 0 on exception, a value greater than N if too long
  */
-#ifndef strnlen_user
+#ifndef __strnlen_user
+#define __strnlen_user strnlen
+#endif
+
 static inline long strnlen_user(const char __user *src, long n)
 {
 	if (!access_ok(VERIFY_READ, src, 1))
 		return 0;
-	return strlen((void * __force)src) + 1;
+	return __strnlen_user(src, n);
 }
-#endif
 
 static inline long strlen_user(const char __user *src)
 {