LU-11526 rpc: support maximum 64MB I/O RPC

LU-11526 rpc: support maximum 64MB I/O RPC

On newer systems, some block drivers allow max_hw_sector_kb to
be up to 65536KB (64MB) to the underlying storage. To maximize
driver efficiency, Lustre should also have bump up maximum I/O
RPC size to 64MB.
Clamp max_read_ahead_whold_mb not to exceed
max_read_ahead_per_file_mb

Signed-off-by: Qian Yingjin <qian@ddn.com>
Change-Id: Icbf78742f8210d82dc310af7d05b7c32b93af34f
Reviewed-on: https://review.whamcloud.com/34042
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Patrick Farrell <pfarrell@whamcloud.com>
Tested-by: Jenkins
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>