neigh: introduce neigh_confirm() helper function

Add neigh_confirm() for the confirmed member in struct neighbour,
it can be called as an independent unit by other functions.

Signed-off-by: Yajun Deng <yajun.deng@linux.dev>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>