Rework key_flush_sad().

Do all work in three steps:

1. Acquire SAHTREE_RLOCK and check that we need to cleanup som SAs. Just return if nothing was found.
2. Acquire SAHTREE_WLOCK and properly unlink SA/SAH from the corresponding lists, change state, etc.
3. With released lock finish cleanup (send SADB_EXPIRE message, release references, etc).