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>
Fix some style and checking issue

Signed-off-by: Xiaoxi Chen <xiaoxi.chen@intel.com>
Merge pull request #2152 from xiaoxichen/fix_ceph_df

PGMonitor: fix bug in caculating pool avail space

Reviewed-by: Sage Weil <sage@redhat.com>