DHT/readdirp: Directory not shown/healed on mount point if exists on single brick(non first up subvolume).

Problem: If snapshot is taken, when mkdir has succeeded only on
hashed_subvolume, then after restoring snapshot the directory
is not shown on mount point.

Why:    dht_readdirp takes only those directory entries in to
account, which are present on first_up_subvolume. Hence, if the
"hashed subvolume" is not same as first_up_subvolume, it wont be listed
on mount point and also not healed.

Solution:
Case 1: (Rebalance not running)If hashed subvolume is NULL or down then
filter in first_up_subvolume. Other wise the corresponding hashed subvolume
will take care of the directory entry.

Case 2: If readdirp_optimize option is turned on then read from first_up_subvol

Change-Id: Idaad28f1c9f688dbfb1a8a3ab8b244510c02365e
BUG: 1139986
Signed-off-by: Susant Palai <spalai@redhat.com>
Reviewed-on: http://review.gluster.org/7599
Reviewed-by: Raghavendra G <rgowdapp@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>
Reviewed-on: http://review.gluster.org/8671
Reviewed-by: Kaleb KEITHLEY <kkeithle@redhat.com>
Tested-by: Kaleb KEITHLEY <kkeithle@redhat.com>