Use union sctp_sockstore instead of struct sockaddr_storage. This
eliminiates some warnings when building in userland.
Thanks to Patrick Laimbock for reporting this issue.
Remove also some unnecessary casts.
There should be no functional change.

MFC after: 1 week


git-svn-id: svn+ssh://svn.freebsd.org/base/head@271221 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f