Remove unused variable sc_tx_bufsz.
The variable is initialized but not used.

Reviewed by:	yongari
Improve locking strategy between keys hash and ID hash.

Before this change state creating sequence was:

1) lock wire key hash
2) link state's wire key
3) unlock wire key hash
4) lock stack key hash
5) link state's stack key
6) unlock stack key hash
7) lock ID hash
8) link into ID hash
9) unlock ID hash

What could happen here is that other thread finds the state via key
hash lookup after 6), locks ID hash and does some processing of the
state. When the thread creating state unblocks, it finds the state
it was inserting already non-virgin.

Now we perform proper interlocking between key hash locks and ID hash
lock:

1) lock wire & stack hashes
2) link state's keys
3) lock ID hash
4) unlock wire & stack hashes
5) link into ID hash
6) unlock ID hash

To achieve that, the following hacking was performed in pf_state_key_attach():

- Key hash mutex is marked with MTX_DUPOK.
- To avoid deadlock on 2 key hash mutexes, we lock them in order determined
  by their address value.
- pf_state_key_attach() had a magic to reuse a > FIN_WAIT_2 state. It unlinked
  the conflicting state synchronously. In theory this could require locking
  a third key hash, which we can't do now.
  Now we do not remove the state immediately, instead we leave this task to
  the purge thread. To avoid conflicts in a short period before state is
  purged, we push to the very end of the TAILQ.
- On success, before dropping key hash locks, pf_state_key_attach() locks
  ID hash and returns.

Tested by:	Ian FREISLICH <ianf clue.co.za>