Do proper rounding in interpolated pressure calculations

Do proper rounding in interpolated pressure calculations

We do all the pressures in mbar, which has plenty of precision for
interpolated pressures - even when we then do our discrete integration
over many samples.

However, when we calculate those interpolated pressure points, we should
make sure that we round the result correctly, otherwise the consistent
rounding errors (from truncating the FP value into our integer mbar
values) will result in a final pressure that is noticeably off in ugly
ways (ie "end pressure set by hand to 750 mbar, but shown as 748").

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>