LU-6910 osp: add procfs values for OST reserved size

LU-6910 osp: add procfs values for OST reserved size

osp_pre_status=-ENOSPC is used to skip OST from object allocation.
The error was set when OST available space is less than 0.1% of total
OST size. This value is not configurable, so procfs files was
added:
reserved_mb_low - low watermark, if available space is less
		  than it, object allocation is stopped.
reserved_mb_high - highw watermark, if available space is more
		   than it, object allocation is enabled.

By default ~0.1% is reserved as low watermark. The high watermark
is twice bigger than the low by default.
High and low watermark could be changed by:
lctl set_param osp.lustre-OST0000-osc-MDT0000.reserved_mb_high=1024

When object allocation is disabled, a clients could appened to
existing files. And 0.1% is too low for them. For example, OST size
is 8TB, 0.1% is 8GB, if cluster has 1k clients, reserved space is
~8MB per client. The main reason of the patch is ability to increase
reserved space.

Signed-off-by: Alexander Boyko <alexander.boyko@seagate.com>
Xyratex-bug-id: MRP-2606
Change-Id: Ie48cc1a232f64aa7dc922000861004277fb47340
Reviewed-on: http://review.whamcloud.com/15731
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Alexander Zarochentsev <alexander.zarochentsev@seagate.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>