ceph: quota: add counter for snaprealms with quota

By keeping a counter with the number of snaprealms that have quota set
allows to optimize the functions that need to walk throught the realms
hierarchy looking for quotas.  Thus, if this counter is zero it's safe to
assume that there are no realms with quota.

Signed-off-by: Luis Henriques <lhenriques@suse.com>
Reviewed-by: "Yan, Zheng" <zyan@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>