selftests: netfilter: Fix nft_fib.sh for all.rp_filter=1

If net.ipv4.conf.all.rp_filter is set, it overrides the per-interface
setting and thus defeats the fix from bbe4c089 ("selftests:
netfilter: disable rp_filter on router"). Unset it as well to cover that
case.

Fixes: bbe4c089 ("selftests: netfilter: disable rp_filter on router")
Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Florian Westphal <fw@strlen.de>