features/quota: Add option to consider the quota limit in statfs estimation

Adds an option, features.quota-deem-statfs (default off) to consider the
quota limits while calculating the volume stats.

Eg: Backend is of size 10GB and limit set on / is 5GB. If the option is off
df show actual size to be 10GB and when it is on df shows 5GB.

Change-Id: Ib30733bb69afecce1dea9d0491af89d551d214cc
BUG: 905425
Signed-off-by: Varun Shastry <vshastry@redhat.com>
Reviewed-on: http://review.gluster.org/4511
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@redhat.com>