LU-4755 ptlrpc: enlarge OST_MAXREQSIZE for 4MB RPC

LU-4755 ptlrpc: enlarge OST_MAXREQSIZE for 4MB RPC

This patch enlarges OST_MAXREQSIZE so as to make the
request size large enough for 4MB RPC.

Signed-off-by: Li Xi <lixi@ddn.com>
Change-Id: I6195c05ced9e6c533c2ad88f21a941f1dddb53e6
Reviewed-on: http://review.whamcloud.com/9599
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Jenkins
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: James Simmons <uja.ornl@gmail.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>