9pnet: trans_fd : allocate struct p9_trans_fd and struct p9_conn together.

There is no point in allocating these structs separately.
Changing this makes the code a little simpler and saves a few bytes of
memory.

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