ceph-mon: check fs stats just before preforking

Otherwise statfs may fail if mkfs hasn't been run yet or if the monitor
data directory does not exist.  There are checks to account for the mon
data dir not existing and we should wait for them to clear before we go
ahead and check the fs stats.

Signed-off-by: Joao Eduardo Luis <joao@redhat.com>
Merge pull request #2557 from ceph/wip-mon-fix-checks

ceph-mon: check fs stats just before preforking

Reviewed-by: Loic Dachary <loic-201408@dachary.org>