PGMonitor: fix bug in caculating pool avail space

Currently for pools with different rules, "ceph df" cannot report
right available space for them, respectively. For detail assisment
of the bug ,pls refer to bug report #8943

This patch fix this bug and make ceph df works correctlly.

Fixes Bug #8943

Signed-off-by: Xiaoxi Chen <xiaoxi.chen@intel.com>
it shouldn't normally happen, but can you guard the osd_stat[] with a check that the osd is present in the osd_stat map? there may be corner cases where it is not and we don't want to implicitly add an empty item to the map in that case.