cluster/dht: Fix min-free-disk calculations when quota-deem-statfs is on

PROBLEM:

As part of file creation, DHT sends a statfs call to all of its
sub-volumes and expects in return the local space consumption and
availability on each one of them. This information is used by DHT to
ensure that atleast min-free-disk amount of space is left on each
sub-volume in the event that there ARE other sub-volumes with more
space available.
But when quota-deem-statfs is enabled, quota xlator on every brick
unwinds the statfs call with volume-wide consumption of disk space.
This leads to miscalculation in min-free-disk algo, thereby misleading
DHT at some point, into thinking all sub-volumes have equal available
space, in which case DHT keeps sending new file creates to subvol-0,
causing it to become 100% full at some point although there ARE other
subvols with ample space available.

FIX:

The fix is to make quota_statfs() behave as if quota xlator weren't
enabled, thereby making every brick return only its local consumption
and disk space availability.

Change-Id: I211371a1eddb220037bd36a128973938ea8124c2
BUG: 1099890
Signed-off-by: Krutika Dhananjay <kdhananj@redhat.com>
Reviewed-on: http://review.gluster.org/7845
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Raghavendra G <rgowdapp@redhat.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>