OSDMonitor: Do not allow OSD removal using setmaxosd

Description: Currently setmaxosd command allows removal of OSDs by providing
a number less than current max OSD number. This causes abrupt removal of
OSDs causing data loss as well as kernel panic when kernel RBDs are involved.
Fix is to avoid removal of OSDs using setmaxosd command.

Fixes: #8865

Signed-off-by: Anand Bhat <anand.bhat@sandisk.com>
I think we can allow this number to shrink as long as OSDMap.exists(osd) == false for each slow between the new and old max.  Then the admin can use 'ceph osd rm ...' to get there...