Add const qualifier to in6_addrhash() function.
Add in6ifa_ifwithaddr() function. It is similar to ifa_ifwithaddr,
but does fast lookup in the hash of inet6 addresses.

Obtained from:	Yandex LLC
Sponsored by:	Yandex LLC


git-svn-id: svn+ssh://svn.freebsd.org/base/head@271427 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f