locks: Don't allow mounts in user namespaces to enable mandatory locking

Since no one uses mandatory locking and files with mandatory locks can
cause problems don't allow them in user namespaces.
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Jeff Layton <jeff.layton@primarydata.com>

fs/namespace.c

@@ -1589,7 +1589,7 @@ static inline bool may_mandlock(void)
 #ifndef	CONFIG_MANDATORY_FILE_LOCKING
 	return false;
 #endif
-	return true;
+	return capable(CAP_SYS_ADMIN);
 }
 
 /*