mon/MDSMonitor: return mon_mds_force_trim_to even if equal to current epoch

mon/MDSMonitor: return mon_mds_force_trim_to even if equal to current epoch

The PaxosService code already excludes the value returned by
PaxosService::get_trim_to as the upper bound of the range of epochs to
trim. Without this fix, you need to set mon_mds_force_trim_to to one
greater than the epoch you want to trim _and_ force the current epoch to
be one greater than that; the net result being that you can only force
trimming up to 2 epochs behind the current epoch.

This change is helpful for resolving issue 51673, but not strictly
necessary.

Related-to: https://tracker.ceph.com/issues/51673
Signed-off-by: Patrick Donnelly <pdonnell@redhat.com>