mds: release free heap pages after trim

mds: release free heap pages after trim

MDS free heap space can grow to large for some workloads (like smallfile
and recursive deletes). This can cause the MDS mapped memory to grow
well beyond memory targets.

When we finally use the PriorityCache in the MDS, this will not be
necessary anymore as the PriorityCache already does this.

Fixes: https://tracker.ceph.com/issues/42938
Signed-off-by: Patrick Donnelly <pdonnell@redhat.com>
(cherry picked from commit af4cac5ec7bab4e5bf8936cd685f6ee8dcb38127)

Conflicts:
    src/mds/MDCache.cc