libxt_statistic: increase precision on create and dump

Currently, libxt_statistic only dumps the probability with a
granularity of 1/1000000. Assuming only stuffed packets with 1440
bytes payload, this would match approximately every 1.341 GB, which is
pretty low for a high-volume router. Trying to match any larger
interval than that (e.g. 2 GB) will cause libxt_statistic to output
"--probability 0.000000", and when restored, will cause it to never
match again.

Bump the dump precision to what xt_statistic can really do, and adjust
the manpage to include a word about it.

Furthermore, employ explicit rounding when reading the argument from
the command line, because the previous implicit conversion would use
truncation, which is not very exact.

Signed-off-by: Jan Engelhardt <jengelh@medozas.de>
2 files changed