LU-9184 grants: move grant code to the target

LU-9184 grants: move grant code to the target

Make grants code common for MDT and OST.
- move grants field from OFD to the lu_target
- same for statfs
- make ofd_grants.c independent from OFD
- move ofd_grants.c to the tgt_grants.c

This work was done for Data-on-MDT feature but it
is needed also for better space control on MDT as
well, e.g. for lfsck, HSM, etc.

Signed-off-by: Mikhal Pershin <mike.pershin@intel.com>
Change-Id: I43930b32b271c9e850aa2783795e7cce12830cd5
Reviewed-on: https://review.whamcloud.com/25820
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Niu Yawei <yawei.niu@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>