rule: allow src/dstnat prios in input and output

rule: allow src/dstnat prios in input and output

commit 8beafab74c391130fbb9111bfccab8613644e3b9 upstream.

Dan Winship says:

The "dnat" command is usable from either "prerouting" or "output", but the
"dstnat" priority is only usable from "prerouting". (Likewise, "snat" is usable
from either "postrouting" or "input", but "srcnat" is only usable from
"postrouting".)

No need to restrict those priorities to pre/postrouting.

Closes: https://bugzilla.netfilter.org/show_bug.cgi?id=1694
Signed-off-by: Florian Westphal <fw@strlen.de>