LU-4933 osc: Automatically increase the max_dirty_mb

LU-4933 osc: Automatically increase the max_dirty_mb

When RPC size or the max RPCs in flight is increased, the actual
limit might be max_dirty_mb. This patch automatically increases
the max_dirty_mb value at connection time and when the related
values are tuned manually by proc file system.

This patch also changes the unit of "cl_dirty" and "cl_dirty_max"
in client_obd from byte to page.

Lustre-change: http://review.whamcloud.com/10446/
Lustre-commit: bdc5bb52c55470cf8020933f80e327c397810603

Signed-off-by: Li Xi <lixi@ddn.com>
Signed-off-by: Hongchao Zhang <hongchao.zhang@intel.com>
Change-Id: I8480122b7370247b17de81d731c2f2b5f67892ce
Reviewed-on: http://review.whamcloud.com/10937
Tested-by: Jenkins
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: James Simmons <uja.ornl@gmail.com>