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: Tadeusz Struk <tadeusz.struk@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/crypto/algif_skcipher.c b/crypto/algif_skcipher.c
index 8276f21..60496d4 100644
--- a/crypto/algif_skcipher.c
+++ b/crypto/algif_skcipher.c
@@ -537,7 +537,7 @@
 
 	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);