MIPS: Fix strnlen_user access check

Commit 04324f44cb ("MIPS: Remove get_fs/set_fs") changed the access_ok
for strnlen_user to check the whole range, which broke some callers
of strndup_user(). Restore the old behaviour and just check the first byte.

Fixes: 04324f44cb ("MIPS: Remove get_fs/set_fs")
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>

arch/mips/include/asm/uaccess.h

@@ -614,8 +614,8 @@ static inline long strnlen_user(const char __user *s, long n)
 {
 	long res;
 
-	if (!access_ok(s, n))
-		return -0;
+	if (!access_ok(s, 1))
+		return 0;
 
 	might_fault();
 	__asm__ __volatile__(