Unlock MMAP thread lock for track dtor

The track dtor now joins a worker thread, which could have tasks on it
which acquire the thread loop lock. To prevent a inversion/deadlock,
drop the thread lock prior to destruction, similar to the other tracks.

Fixes: 411337010
Flag: EXEMPT bugfix
Test: oboetester mmap
Change-Id: Icb518864c57d7cb53966655ffcf69c16a17facd5