netfilter: nf_tables: Drop pointless memset when dumping rules

None of the dump callbacks uses netlink_callback::args beyond the first
element, no need to zero the data.

Fixes: 96518518 ("netfilter: add nftables")
Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Florian Westphal <fw@strlen.de>