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>
(cherry picked from commit 7f71c11666b25e91dd612c58b4eda9ac0d4752f8)

Conflicts:
	src/ceph_mon.cc