build_image too aggressive with right sizing

Regression introduced by c3d408e3

Found a case where 1 + 1/59 reduction in size based on free space
failed when right sizing an ext4 image. Reduced the math to 1 + 1/60.

Test: compile
Bug: 111302946
Change-Id: I6a0f3919e8d45d0b3e7cd558a6ad4d3799012f2c