netinet6: Fix memory leak on auto_linklocal

release the refcount of link-local prefix information to ensure
it gets freed when the address is deleted.

Reviewed By: zlei, ivy
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D55593

(cherry picked from commit b55bffeaaf9bae5dc7aa21eae441d89c999ebab8)