Simplify bridge(4)

[Ryota Ozaki <ozaki-r%netbsd.org@localhost>]

Hi,

Thanks to introducing softint-based if_input,
we can simplify bridge(4).

- Remove spin mutexes
  - They were needed because some code of bridge could run in
    hardware interrupt context
  - We now need only an adaptive mutex for each shared object
    (a member list and a forwarding table)
- Remove pktqueue
  - bridge_input is already in softint, using another softint
    (for bridge_forward) is useless
  - Packet distribution should be down at device drivers

As expected, forwarding performance improves slightly
because of stopping using the second softint.

Here is a patch:
http://www.netbsd.org/~ozaki-r/simplify-bridge.diff

Any comments or suggestions?

Thanks,
  ozaki-r