LU-3285 mdt: use generic grant code at MDT

LU-3285 mdt: use generic grant code at MDT

Use grants at MDT for Data-on-MDT needs.
Add parameter to reserve part of available space
for metadata and never grants it to clients.

Test-Parameters: mdssizegb=20 testlist=dom-performance
Signed-off-by: Mikhal Pershin <mike.pershin@intel.com>
Change-Id: I2612352062871e4edd3817f32e7d96cb95a0a52b
Reviewed-on: https://review.whamcloud.com/28021
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>