um: return negative in tuntap_open_tramp()

The intention is to return negative error codes. "pid" is already
negative but we accidentally negate it again back to positive.

Signed-off-by: Dan Carpenter <[e-mail]>
Signed-off-by: Richard Weinberger <[e-mail]>

arch/um/os-Linux/drivers/tuntap_user.c

···
 	pid = run_helper(tuntap_pre_exec, &data, argv);
 
 	if (pid < 0)
-		return -pid;
+		return pid;
 
 	close(remote);