9pfs: fix error path in pdu_submit()

If we receive an unsupported request id, we first decide to
return -ENOTSUPP to the client, but since the request id
causes is_read_only_op() to return false, we change the
error to be -EROFS if the fsdev is read-only. This doesn't
make sense since we don't know what the client asked for.

This patch ensures that -EROFS can only be returned if the
request id is supported.

Signed-off-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Eric Blake <eblake@redhat.com>

hw/9pfs/9p.c

@@ -3473,14 +3473,12 @@ void pdu_submit(V9fsPDU *pdu, P9MsgHeader *hdr)
     if (pdu->id >= ARRAY_SIZE(pdu_co_handlers) ||
         (pdu_co_handlers[pdu->id] == NULL)) {
         handler = v9fs_op_not_supp;
+    } else if (is_ro_export(&s->ctx) && !is_read_only_op(pdu)) {
+        handler = v9fs_fs_ro;
     } else {
         handler = pdu_co_handlers[pdu->id];
     }
 
-    if (is_ro_export(&s->ctx) && !is_read_only_op(pdu)) {
-        handler = v9fs_fs_ro;
-    }
-
     qemu_co_queue_init(&pdu->complete);
     co = qemu_coroutine_create(handler, pdu);
     qemu_coroutine_enter(co);