inet6: protect address manipulation with a lock

This is a total hack/bare minimum which follows inet4.

Otherwise 2 threads removing the same address can easily crash.

Reviewed by: kp
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D39317

(cherry picked from commit f5a365e51feea75d1e5ebc86c53808d8cae7b6d7)