mds: check metadata pool not cluster is full

mds: check metadata pool not cluster is full

CEPH_OSDMAP_FULL flag was obsoleted by
b4ca5ae462c6f12ca48b787529938862646282cd. So, check if the metadata pool is
full instead which is a decent proxy (as metadata operations can still
proceed). However, the data pool may still be full which would result in some
operations still not completing (like inode backtrace updates).

Fixes: http://tracker.ceph.com/issues/22483
Signed-off-by: Patrick Donnelly <pdonnell@redhat.com>
(cherry picked from commit d678415bb03afd1a67edaa0eac031a6a9cf3fbf9)