ceph: quota: add initial infrastructure to support cephfs quotas

This patch adds the infrastructure required to support cephfs quotas as it
is currently implemented in the ceph fuse client.  Cephfs quotas can be
set on any directory, and can restrict the number of bytes or the number
of files stored beneath that point in the directory hierarchy.

Quotas are set using the extended attributes 'ceph.quota.max_files' and
'ceph.quota.max_bytes', and can be removed by setting these attributes to
'0'.

Link: http://tracker.ceph.com/issues/22372
Signed-off-by: Luis Henriques <lhenriques@suse.com>
Reviewed-by: "Yan, Zheng" <zyan@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>