crypto: algif - fix warn: unsigned 'used' is never less than zero

Change type from unsigned long to int to fix an issue reported by kbuild robot:
crypto/algif_skcipher.c:596 skcipher_recvmsg_async() warn: unsigned 'used' is
never less than zero.
Signed-off-by: NTadeusz Struk <tadeusz.struk@intel.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

crypto/algif_skcipher.c

@@ -537,7 +537,7 @@ static int skcipher_recvmsg_async(struct socket *sock, struct msghdr *msg,
 
 	while (iov_iter_count(&msg->msg_iter)) {
 		struct skcipher_async_rsgl *rsgl;
-		unsigned long used;
+		int used;
 
 		if (!ctx->used) {
 			err = skcipher_wait_for_data(sk, flags);