MFC r256833:

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>
Approved by: re (hrs)