cluster/dht: pathinfo xattr changes for directories

Since directories have presence on all subvolumes there is
no definite meaning of ->hashed_subvol or ->cached_subvol.
getxattr() code path chooses ->cached_subvol for pathinfo
extended attribute. While this makes sense of files, it makes
less sense for directories. Further if a hashed or a cached
subvolume is down, and there's a getxattr request for a
directory, we return with an errno.

This patch changes pathinfo extended attribute contents by
aggregating information from all subvolumes that are up.

Change-Id: I58adb741d63ccfd1d0239af75eb65f26f0fb384d
Signed-off-by: Venky Shankar <vshankar@redhat.com>
BUG: 856455
Reviewed-on: http://review.gluster.org/4047
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@redhat.com>