LU-12612 osd: add lnb size down to osd

LU-12612 osd: add lnb size down to osd

so that each OSD can check for lnb array overflow.
the patch isn't final - there will be proper
implementation in osd-zfs and a new test.

Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Change-Id: I43683c84e48006b4075f9a8b3e87cdfeae28c02b
Reviewed-on: https://review.whamcloud.com/35801
Reviewed-by: Mike Pershin <mpershin@whamcloud.com>
Reviewed-by: Lai Siyao <lai.siyao@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>