fix: Rework FAT size calculation for volume formatting
fix: Rework FAT size calculation for volume formatting

New formula gives nearly perfect results (according to tests) and is
better than formula from specification.
Also all math transformations are provided in comments so it is no longer
a blindly copied formula from spec but a Math supported formula.
Old formula was not taking into account sector size (it resulted in too
big FATs for sector size different than 512) and had rounding issues for
FAT12 and FAT32.

rafalh/rust-fat,rafalh/rust-fat