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>
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