9pfs: improve v9fs_walk() tracing

9pfs: improve v9fs_walk() tracing

'Twalk' is the most important request type in the 9p protocol to look out
for when debugging 9p communication. That's because it is the only part
of the 9p protocol which actually deals with human-readable path names,
whereas all other 9p request types work on numeric file IDs (FIDs) only.

Improve tracing of 'Twalk' requests, e.g. let's say client wanted to walk
to "/home/bob/src", then improve trace output from:

  v9fs_walk tag 0 id 110 fid 0 newfid 1 nwnames 3

to:

  v9fs_walk tag=0 id=110 fid=0 newfid=1 nwnames=3 wnames={home, bob, src}

To achieve this, add a new helper function trace_v9fs_walk_wnames() which
converts the received V9fsString array of individual path elements into a
comma-separated string presentation for being passed to the tracing system.
As this conversion is somewhat expensive, this conversion function is only
called if tracing of event 'v9fs_walk' is currently enabled.

Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
Reviewed-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <E1tJamT-007Cqk-9E@kylie.crudebyte.com>