src: disallow burst 0 in ratelimits

src: disallow burst 0 in ratelimits

The ratelimiter in nftables is similar to the one in iptables, and
iptables disallows a zero burst.

Update the byte rate limiter not to print burst 5 (default value).

Update tests/py payloads to print burst 5 instead of zero when the
burst is unspecified.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>