Test arp_ndisc_untracked_subnets use tcpdump to filter the unsolicited
and untracked na messages. It set -e before calling tcpdump. But if
tcpdump filters 0 packet, it will return none zero, and cause the script
to exit.

Instead of using slow tcpdump to capture packets, let's using tc rule
to filter out the na message.

At the same time, fix function setup_v6 which only needs one parameter.
Move all the related helpers from forwarding lib.sh to net lib.sh.

Fixes: 0ea7b0a4 ("selftests: net: arp_ndisc_untracked_subnets: test for arp_accept and accept_untracked_na")
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://lore.kernel.org/r/20240517010327.2631319-1-liuhangbin@gmail.comSigned-off-by: Paolo Abeni <pabeni@redhat.com>