BUG/MEDIUM: sessions: Always pass the mux context as argument to destroy a mux

This bug was introduced by the commit 2444aa5b ("MEDIUM: sessions: Don't be
responsible for connections anymore."). In session_check_idle_conn(), when the
mux is destroyed, its context must be passed as argument instead of the
connection.

It is de 2.2-dev bug. No need to backport.
diff --git a/include/proto/session.h b/include/proto/session.h
index b6f31f1..a5b95a8 100644
--- a/include/proto/session.h
+++ b/include/proto/session.h
@@ -126,7 +126,7 @@
 		session_unown_conn(sess, conn);
 		conn->owner = NULL;
 		conn->flags &= ~CO_FL_SESS_IDLE;
-		conn->mux->destroy(conn);
+		conn->mux->destroy(conn->ctx);
 		return -1;
 	} else {
 		conn->flags |= CO_FL_SESS_IDLE;