netinet6: Tear down IPv6 source address selection policies with rest of IPv6.

This may plug minor leaks which no-one has reported. The default IPv6 source
address selection policy list in FreeBSD is usually limited to 9 entries,
and can be readily inspected with ip6addrctl(8). The policy table is
however instantiated for each VNET.

The leak of a pol instance in delete_addrsel_policyent() was already
plugged by @ae in commit-id ecc5c73, so that change has not been merged.

Do not tear down the sxlocks as glebius has requested, and move the
addrsel_policyent{} declarations further up to avoid redundant forward
declarations as glebius requested for stylistic reasons.

Reviewed by: ae, pouria
Sponsored by: Cisco Systems, Inc.
Differential Revision: https://reviews.freebsd.org/D55599