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>
Yeah.  Can simplify this quite a bit though:

- can drop the osd_exists variable entirely and put the err = .. and goto inside the inner if condition
- the outer if is unnecessary as the for loop condition checks the same thing
- "== true" is redundant.. just "if (osdmap.exists(i))"
- make sure the indentation is consistent

Then it is just a single for loop with an inner if. Thanks!