BUG/MEDIUM: checks: Increment the server's curr_used_conns

In tcpcheck_eval_connect(), if we're targetting a server, increase its
curr_used_conns when creating a new connection, as the counter will be
decreased later when the connection is destroyed and conn_free() is called.
diff --git a/src/tcpcheck.c b/src/tcpcheck.c
index 12d7451..4b8806c 100644
--- a/src/tcpcheck.c
+++ b/src/tcpcheck.c
@@ -1026,7 +1026,11 @@
 
 	/* Maybe there were an older connection we were waiting on */
 	check->wait_list.events = 0;
-	conn->target = s ? &s->obj_type : &proxy->obj_type;
+	if (s) {
+		_HA_ATOMIC_ADD(&s->curr_used_conns, 1);
+		conn->target = &s->obj_type;
+	} else
+		conn->target = &proxy->obj_type;
 
 	/* no client address */
 	if (!sockaddr_alloc(&conn->dst)) {