RDMA/nes: Remove session_id from nes_cm stuff

The session_id members of struct nes_cm_listener and struct
nes_cm_node are write-only, so remove them.  This allows the
session_id member of struct nes_cm_core to be removed as well, since
it is only used to write those other session_id values.

This removes the use of current->tgid (which will be deprecated)
pointed out by Pavel Emelyanov <xemul@openvz.org>.

Acked-by: Glenn Streiff <gstreiff@neteffect.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>