vxlan: Move address helpers to private headers

Move the helpers out of the core C file to the private header so that
they could be used by the upcoming MDB code.

While at it, constify the second argument of vxlan_nla_get_addr().

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Nikolay Aleksandrov <razor@blackwall.org>
Signed-off-by: David S. Miller <davem@davemloft.net>