infiniband: addr: Consolidate code to fetch neighbour hardware address from dst.

IPV4 should do exactly what the IPV6 code does here, which is
use the neighbour obtained via the dst entry.

And now that the two code paths do the same thing, use a common
helper function to perform the operation.

Signed-off-by: David S. Miller <davem@davemloft.net>
Acked-by: Eric Dumazet <eric.dumazet@gmail.com>
Acked-by: Roland Dreier <roland@purestorage.com>