neigh: Consider neighbor entries for multipath routes
neigh: Consider neighbor entries for multipath routes

Use status of existing neighbor entry when determining nexthop for multipath
routes. If disabled, neighbor information is not used and packets could be
directed to a failed nexthop which is the default behavior by the kernel for
compat reasons. During the discussion around managed neighbor entries, the
question came up on whether we make use of fib_multipath_use_neigh. Enabling
this setting is definitely recommended and will help bpf_fib_lookup() helper
in our datapath.

Link: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=a6db4494d218c2e559173661ee972e048dc04fdd
Signed-off-by: Daniel Borkmann <3d0f3b9ddcacec30c4008c5e030e6c13a478cb4f@iogearbox.net>

cilium/cilium,cilium/cilium,cilium/cilium,michi-covalent/cilium,michi-covalent/cilium,michi-covalent/cilium,tklauser/cilium,tklauser/cilium,tklauser/cilium,cilium/cilium,cilium/cilium,tklauser/cilium,tklauser/cilium,michi-covalent/cilium,michi-covalent/cilium