LU-10990 osc: increase default max_dirty_mb to 2G

LU-10990 osc: increase default max_dirty_mb to 2G

While ideally we want to go away from max_dirty_mb setting
completely and let grants code to take the msot part of it,
Andreas raises a somewhat valid point that for certain
system configurations with high-latency links, system
administrators might want to have ability to limit
amount of dirty pages just for those OSCs to limit amount
of time it might take to flush that dirty data.

So a good compromise is to lift the max_dirty_mb default
value first while we work out the current grant code
deficiencies

Change-Id: I4de407088af70e0f98f0563160217ba70a635dfb
Signed-off-by: Oleg Drokin <oleg.drokin@intel.com>
Reviewed-on: https://review.whamcloud.com/32288
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Patrick Farrell <paf@cray.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>