mld: always commit state changes on leaving

Resolve a race condition where we'd lose the Solicited-node multicast
group subscription if we assigned the same IPv6 address twice.

PR: 233683
Reviewed by: ae
MFC after: 1 week
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D41124

(cherry picked from commit 9c9a76dc6873427b14f6c84397dd60ea8e529d8d)