9pfs: reduce latency of Twalk

9pfs: reduce latency of Twalk

As with previous performance optimization on Treaddir handling;
reduce the overall latency, i.e. overall time spent on processing
a Twalk request by reducing the amount of thread hops between the
9p server's main thread and fs worker thread(s).

In fact this patch even reduces the thread hops for Twalk handling
to its theoritical minimum of exactly 2 thread hops:

main thread -> fs worker thread -> main thread

This is achieved by doing all the required fs driver tasks altogether
in a single v9fs_co_run_in_worker({ ... }); code block.

Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
Reviewed-by: Greg Kurz <groug@kaod.org>
Message-Id: <1a6701674afc4f08d40396e3aa2631e18a4dbb33.1622821729.git.qemu_oss@crudebyte.com>