net/9p: fix memory handling/allocation in rdma_request()

Return -ENOMEM when erroring on kmalloc and fix memory leaks when returning on error.

Signed-off-by: Davidlohr Bueso <dave@gnu.org>
Reviewed-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Eric Van Hensbergen <ericvh@gmail.com>