build_image: right-sizing at least some spare inode

To address problems creating real tiny filesystem placeholders,
make sure extra spare inodes margin is greater than 0.
For initial estimate we add 4% of total we request at least 8.
For second pass estimate we add 0.2% of total we request at least 1.

We bumped up the margin for zie on the second pass to 0.3% as the
value was too close on one of the builds.

Test: build
Bug: 122328872
Change-Id: I41707bb6fcc8bbfbdda143a9ce62446cad9c1533