net: pppoe - code cleanup and helpers

- Introduce PPPOE_HASH_MASK.
- Remove redundant declaration of pppoe_chan_ops.
- Introduce stage_session helper.
- Tabs, space, long-line-split cleanup.

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>