LU-7990 llite: increase whole-file readahead to RPC size

LU-7990 llite: increase whole-file readahead to RPC size

Increase the default whole-file readahead limit to match the current
RPC size.  That ensures that files smaller than the RPC size will be
read in a single round-trip instead of sending multiple smaller RPCs.

Signed-off-by: Andreas Dilger <andreas.dilger@intel.com>
Change-Id: I3bdb1c7f92c546d58951a9e6b783af23c83ebbe5
Reviewed-on: https://review.whamcloud.com/26955
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Patrick Farrell <paf@cray.com>
Reviewed-by: Dmitry Eremin <dmitry.eremin@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>