fix default __strnlen_user macro

The existing __strnlen_user macro simply resolved to strnlen. However, the
count returned by strnlen_user should include the NULL byte. This patch
fixes the __strnlen_user macro to include the NULL byte in the count.
Signed-off-by: Mark Salter <msalter@redhat.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>

include/asm-generic/uaccess.h

@@ -289,9 +289,14 @@ strncpy_from_user(char *dst, const char __user *src, long count)
  * Return 0 on exception, a value greater than N if too long
  */
 #ifndef __strnlen_user
-#define __strnlen_user strnlen
+#define __strnlen_user(s, n) (strnlen((s), (n)) + 1)
 #endif
 
+/*
+ * Unlike strnlen, strnlen_user includes the nul terminator in
+ * its returned count. Callers should check for a returned value
+ * greater than N as an indication the string is too long.
+ */
 static inline long strnlen_user(const char __user *src, long n)
 {
 	if (!access_ok(VERIFY_READ, src, 1))