conntrack: Fix FTP seq_skew boundary adjustments.

At the same time, splice out a function and also rely on the compiler
for overflow/underflow handling.

Found by inspection.

Fixes: bd5e81a ("Userspace Datapath: Add ALG infra and FTP.")
Signed-off-by: Darrell Ball <dlu998@gmail.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>

lib/conntrack.c

@@ -3173,6 +3173,13 @@ repl_ftp_v6_addr(struct dp_packet *pkt, struct ct_addr v6_addr_rep,
     return overall_delta;
 }
 
+/* Increment/decrement a TCP sequence number. */
+static void
+adj_seqnum(ovs_16aligned_be32 *val, int32_t inc)
+{
+    put_16aligned_be32(val, htonl(ntohl(get_16aligned_be32(val)) + inc));
+}
+
 static void
 handle_ftp_ctl(struct conntrack *ct, const struct conn_lookup_ctx *ctx,
                struct dp_packet *pkt, const struct conn *ec, long long now,
@@ -3247,34 +3254,9 @@ handle_ftp_ctl(struct conntrack *ct, const struct conn_lookup_ctx *ctx,
     struct tcp_header *th = dp_packet_l4(pkt);
 
     if (nat && ec->seq_skew != 0) {
-        if (ctx->reply != ec->seq_skew_dir) {
-
-            uint32_t tcp_ack = ntohl(get_16aligned_be32(&th->tcp_ack));
-
-            if ((ec->seq_skew > 0) && (tcp_ack < ec->seq_skew)) {
-                /* Should not be possible; will be marked invalid. */
-                tcp_ack = 0;
-            } else if ((ec->seq_skew < 0) &&
-                       (UINT32_MAX - tcp_ack < -ec->seq_skew)) {
-                tcp_ack = (-ec->seq_skew) - (UINT32_MAX - tcp_ack);
-            } else {
-                tcp_ack -= ec->seq_skew;
-            }
-            ovs_be32 new_tcp_ack = htonl(tcp_ack);
-            put_16aligned_be32(&th->tcp_ack, new_tcp_ack);
-        } else {
-            uint32_t tcp_seq = ntohl(get_16aligned_be32(&th->tcp_seq));
-            if ((ec->seq_skew > 0) && (UINT32_MAX - tcp_seq < ec->seq_skew)) {
-                tcp_seq = ec->seq_skew - (UINT32_MAX - tcp_seq);
-            } else if ((ec->seq_skew < 0) && (tcp_seq < -ec->seq_skew)) {
-                /* Should not be possible; will be marked invalid. */
-                tcp_seq = 0;
-            } else {
-                tcp_seq += ec->seq_skew;
-            }
-            ovs_be32 new_tcp_seq = htonl(tcp_seq);
-            put_16aligned_be32(&th->tcp_seq, new_tcp_seq);
-        }
+        ctx->reply != ec->seq_skew_dir ?
+            adj_seqnum(&th->tcp_ack, -ec->seq_skew) :
+            adj_seqnum(&th->tcp_seq, ec->seq_skew);
     }
 
     th->tcp_csum = 0;