libnftables: get rid of repeated initialization of netlink_ctx

libnftables: get rid of repeated initialization of netlink_ctx

Most members in the context don't change, so there is no need to memset
it and reassign them on every iteration.  Moved that code out of the
loop.

Fixes: a72315d2bad4 ("src: add rule batching support")
Reported-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Jeremy Sowden <jeremy@azazel.net>
Acked-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>