LU-4933 osc: Automatically tune the max_dirty_mb

LU-4933 osc: Automatically tune the max_dirty_mb

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

Signed-off-by: Li Xi <lixi@ddn.com>
Change-Id: I95e3e32294a96c4022bcefe392d375583f879b2e
Reviewed-on: http://review.whamcloud.com/10034
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>