[net/9p] Assign type of transaction to tc->pdu->id which is otherwise unsed.

This will be used by the transport layer to determine the out going
request type. Transport layer uses this information to correctly
place the mapped pages in the PDU. Patches following this will make
use of this to achieve zero copy.

Signed-off-by: Venkateswararao Jujjuri <jvrao@linux.vnet.ibm.com>
Signed-off-by: Eric Van Hensbergen <ericvh@gmail.com>