9p: Rename req to rreq in trans_fd

In struct p9_conn, rename req to rreq as it is used by the read routine.

Link: http://lkml.kernel.org/r/20180903160321.2181-1-tomasbortoli@gmail.com
Signed-off-by: Tomas Bortoli <tomasbortoli@gmail.com>
Suggested-by: Jun Piao <piaojun@huawei.com>
Signed-off-by: Dominique Martinet <dominique.martinet@cea.fr>