Fix free space map to correctly track the total amount of FSM space needed

Fix free space map to correctly track the total amount of FSM space needed
even when a single relation requires more than max_fsm_pages pages.  Also,
make VACUUM emit a warning in this case, since it likely means that VACUUM
FULL or other drastic corrective measure is needed.  Per reports from Jeff
Frost and others of unexpected changes in the claimed max_fsm_pages need.