LU-3421 grant: more aggressively book space for precreate

LU-3421 grant: more aggressively book space for precreate

- ofd_grant() should not round sub-block allocation to 0.
- ofd_grant_create() should be more aggressive in reserving space.
  Precreation should try to always have enough grant space to process
  a very large preallocation request.

Signed-off-by: Johann Lombardi <johann.lombardi@intel.com>
Change-Id: I5803b2d5fae216edcaae124d3b50b589c02e7251
Reviewed-on: http://review.whamcloud.com/6546
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: James Simmons <uja.ornl@gmail.com>