net: vxlan: add macro definition for number of IANA VXLAN-GPE port

Add macro definition for number of IANA VXLAN-GPE port for generic use.

Signed-off-by: Hao Chen <chenhao288@hisilicon.com>
Signed-off-by: Guangbin Huang <huangguangbin2@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>