glusterd/quota: Heal pgfid xattr on existing data when the quota is enable

This is a backport of http://review.gluster.org/#/c/8878/

The pgfid extended attributes are used to construct the ancestry path
(from the file to the volume root) for nameless lookups on files.
As NFS relies on nameless lookups heavily, quota enforcement through NFS
would be inconsistent if quota were to be enabled on a volume with
existing data.

Solution is to heal the pgfid extended attributes as a part of lookup
perfomed by quota-crawl process. In a posix lookup check for pgfid xattr
and if it is missing set the xattr.

BUG: 1147953
Change-Id: I707d91a056e07452bfd1e070af5eddaa752a84ac
Signed-off-by: vmallika <vmallika@redhat.com>
Reviewed-on: http://review.gluster.org/8890
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>