netfilter: xt_recent: Lift restrictions on max hitcount value

Support tracking of up to 65535 packets per table entry instead of just
255 to better facilitate longer term tracking or higher throughput
scenarios.

Note how this aligns sizes of struct recent_entry's 'nstamps' and
'index' fields when 'nstamps' was larger before. This is unnecessary as
the value of 'nstamps' grows along with that of 'index' after being
initialized to 1 (see recent_entry_update()). Its value will thus never
exceed that of 'index' and therefore does not need to provide space for
larger values.

Requested-by: Fabio <pedretti.fabio@gmail.com>
Link: https://bugzilla.netfilter.org/show_bug.cgi?id=1745


Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>