read-ahead: re-enable support for variable page size

Support for variable size page-size was disabled with the introduction
of fixed size iobufs. Since the introduction of variable sized iobufs
there is no reason to not have configurable page-size in read-ahead.

This patch enables necessary changes in the translator for configurable
page-size.

Change-Id: I677d70fef50641eb041269aca92a088b9d4961cc
BUG: 764204
Signed-off-by: Anand Avati <avati@redhat.com>
Reviewed-on: http://review.gluster.org/4526
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Raghavendra G <raghavendra@gluster.com>