9pfs: fix crash in v9fs_walk()

9pfs: fix crash in v9fs_walk()

v9fs_walk() utilizes the v9fs_co_run_in_worker({...}) macro to run the
supplied fs driver code block on a background worker thread.

When either the 'Twalk' client request was interrupted or if the client
requested fid for that 'Twalk' request caused a stat error then that
fs driver code block was left by 'break' keyword, with the intention to
return from worker thread back to main thread as well:

    v9fs_co_run_in_worker({
        if (v9fs_request_cancelled(pdu)) {
            err = -EINTR;
            break;
        }
        err = s->ops->lstat(&s->ctx, &dpath, &fidst);
        if (err < 0) {
            err = -errno;
            break;
        }
        ...
    });

However that 'break;' statement also skipped the v9fs_co_run_in_worker()
macro's final and mandatory

    /* re-enter back to qemu thread */
    qemu_coroutine_yield();

call and thus caused the rest of v9fs_walk() to be continued being
executed on the worker thread instead of main thread, eventually
leading to a crash in the transport virtio transport driver.

To fix this issue and to prevent the same error from happening again by
other users of v9fs_co_run_in_worker() in future, auto wrap the supplied
code block into its own

    do { } while (0);

loop inside the 'v9fs_co_run_in_worker' macro definition.

Full discussion and backtrace:
https://lists.gnu.org/archive/html/qemu-devel/2021-08/msg05209.html
https://lists.gnu.org/archive/html/qemu-devel/2021-09/msg00174.html

Fixes: 8d6cb100731c4d28535adbf2a3c2d1f29be3fef4
Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
Cc: qemu-stable@nongnu.org
Reviewed-by: Greg Kurz <groug@kaod.org>
Message-Id: <E1mLTBg-0002Bh-2D@lizzy.crudebyte.com>