This commit changes statfs default behaviour when reporting usage
statistics.  Instead of using the overall filesystem usage, statfs now
reports the quota for the filesystem root, if ceph.quota.max_bytes has
been set for this inode.  If quota hasn't been set, it falls back to the
old statfs behaviour.

A new mount option is also added ('noquotadf') to disable this behaviour.
Signed-off-by: Luis Henriques <lhenriques@suse.com>
Reviewed-by: "Yan, Zheng" <zyan@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>