selftests: net: add test for IPv4 route with loopback IPv6 nexthop

selftests: net: add test for IPv4 route with loopback IPv6 nexthop

Add a regression test for a kernel panic that occurs when an IPv4 route
references an IPv6 nexthop object created on the loopback device.

The test creates an IPv6 nexthop on lo, binds an IPv4 route to it, then
triggers a route lookup via ping to verify the kernel does not crash.

  ./fib_nexthops.sh
  Tests passed: 249
  Tests failed:   0

Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Jiayuan Chen <jiayuan.chen@shopee.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://patch.msgid.link/20260304113817.294966-3-jiayuan.chen@linux.dev
Signed-off-by: Jakub Kicinski <kuba@kernel.org>