Add a note that lacp_compose_key() should be updated, when new media
types will be added.

Submitted by:	melifaro
X-MFC after:	r256689
Drop cm_lock before calling mapper_close, which in turn could call
_citrus_mapper_close again and result in a deadlock otherwise.

This is similar to NetBSD PR/24023 (fixed in their r1.5 of this file).

PR:		bin/182994
Submitted by:	Fabian Keil <fk fabiankeil de>
MFC after:	3 days