staging: lustre: 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>
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-4755
Reviewed-on: http://review.whamcloud.com/9599


Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>
Reviewed-by: James Simmons <uja.ornl@gmail.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>
Signed-off-by: James Simmons <jsimmons@infradead.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>