selftests: netfilter: add nft_fib_nexthop test

selftests: netfilter: add nft_fib_nexthop test

Functional coverage of nft_fib6_eval()'s nexthop enumeration over
three route shapes:

  1) single external nexthop (nhid)
  2) external nexthop group (nhid -> group)
  3) old-style multipath (nexthop ... nexthop ...)

Each scenario places one nexthop on the input device (veth0). For
(2) and (3) the matching nexthop is the second member, so the walk
has to traverse beyond the primary nh. Two nft counters on prerouting
verify the data path: one increments only when fib reports veth0 as
the oif, the other counts "missing" results and must stay at zero.

  ./nft_fib_nexthop.sh
  PASS: single external nexthop (nhid -> veth0)
  PASS: nexthop group (dummy0 + veth0)
  PASS: old-style multipath (sibling on veth0)

Suggested-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
Signed-off-by: Florian Westphal <fw@strlen.de>