9p-trans_fd: fix and clean up module init/exit paths

trans_fd leaked p9_mux_wq on module unload.  Fix it.  While at it,
collapse p9_mux_global_init() into p9_trans_fd_init().  It's easier to
follow this way and the global poll_tasks array is about to removed
anyway.

Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Eric Van Hensbergen <ericvh@gmail.com>