[Orc] Fix the SharedMemoryMapper dtor

As briefly discussed on https://reviews.llvm.org/rG1134d3a03facccd75efc5385ba46918bef94fcb6, fix the unintended copy while iterating on Reservations and add a mutex guard, to be symmetric with other usages of Reservations.

Differential revision: https://reviews.llvm.org/D134212