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

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.

Change-Id: I5912ea96787625c496bde56d43ac9162596032e9
BUG: 1147378
Signed-off-by: vmallika <vmallika@redhat.com>
Reviewed-on: http://review.gluster.org/8878
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>