client: report root's quota in statfs

client: report root's quota in statfs

When user is mounted a quota-restricted inode
as the root, report that inode's quota status
as the filesystem statistics in statfs.

This allows us to have a fairly convincing illusion
that someone has a filesystem to themselves, when
they're really mounting a restricted part of
the larger global filesystem.

Fixes: http://tracker.ceph.com/issues/15599
Signed-off-by: John Spray <john.spray@redhat.com>