9P: Add cancelled() to the transport functions.

And move transport-specific code out of net/9p/client.c

Signed-off-by: Simon Derr <simon.derr@bull.net>
Signed-off-by: Eric Van Hensbergen <ericvh@gmail.com>

diff --cc include/net/9p/transport.h
Simple merge

diff --cc net/9p/client.c
Simple merge

diff --cc net/9p/trans_fd.c
Simple merge