malloc debug: reset FreeTrackData mutex after fork

Lock the FreeTrackData mutex during fork and reset it in the child.
Ensures that the FreeTrackData is consistent when forking from a
multi-threaded process, and that the mutex is in a defined state
in the child.

Test: 89hours MTBF test
Change-Id: I5e5892832a733ea85727ec65abc7094d95a725ef