net/9p: use system_wq instead of p9_mux_wq

With cmwq, there's no reason to use a dedicated workqueue in trans_fd.
Drop p9_mux_wq and use system_wq instead.  The used work items are
already sync canceled in p9_conn_destroy() and doesn't require further
synchronization.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Eric Van Hensbergen <ericvh@gmail.com>
Cc: Ron Minnich <rminnich@sandia.gov>
Cc: Latchesar Ionkov <lucho@ionkov.net>
Cc: v9fs-developer@lists.sourceforge.net