cluster/afr: Fix sizeof typo

Change-Id: Ib82a1c4967f0880c91c114e4baae08bdbe77bb60
BUG: 1153626
Signed-off-by: Pranith Kumar K <pkarampu@redhat.com>
Reviewed-on: http://review.gluster.org/8935
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Niels de Vos <ndevos@redhat.com>