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>