x86_64: Fix strnlen_user() to not touch memory after specified maximum

Inspired by commit f18c34e4 ("lib: Fix strnlen_user() to not touch
memory after specified maximum") upstream.  This version of
strnlen_user(), no longer present upstream, has a similar off-by-one
error.
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Cc: Jan Kara <jack@suse.cz>

arch/x86/lib/usercopy_64.c

@@ -113,7 +113,7 @@ long __strnlen_user(const char __user *s, long n)
 	char c;
 
 	while (1) {
-		if (res>n)
+		if (res >= n)
 			return n+1;
 		if (__get_user(c, s))
 			return 0;