Rename ip4_def_policy variable to def_policy. It is used by both IPv4 and
IPv6. Initialize it only once in def_policy_init(). Remove its
initialization from key_init() and make it static.

Remove several fields from struct secpolicy:

• lock - it isn't so useful having mutex in the structure, but the only thing we do with it is initialization and destroying.
• state - it has only two values - DEAD and ALIVE. Instead of take a lock and change the state to DEAD, then take lock again in GC function and delete policy from the chain - keep in the chain only ALIVE policies.
• scangen - it was used in GC function to protect from sending several SADB_SPDEXPIRE messages for one SPD entry. Now we don't keep DEAD entries in the chain and there is no need to have scangen variable.

Use TAILQ to implement SPD entries chain. Use rmlock to protect access
to SPD entries chain. Protect all SP lookup with RLOCK, and use WLOCK
when we are inserting (or removing) SP entry in the chain.

Instead of using pattern "LOCK(); refcnt++; UNLOCK();", use refcount(9)
API to implement refcounting in SPD. Merge code from key_delsp() and
_key_delsp() into _key_freesp(). And use KEY_FREESP() macro in all cases
when we want to release reference or just delete SP entry.

Obtained from: Yandex LLC
Sponsored by: Yandex LLC