cluster/afr: Set size based source only when sizes are unequal

Change-Id: I18583f14edf1011401be15744371e2a6b79d75cc
BUG: 1003842
Signed-off-by: Pranith Kumar K <pkarampu@redhat.com>
Reviewed-on: http://review.gluster.org/5763
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@redhat.com>