net: Add IANA_VXLAN_UDP_PORT definition to vxlan header file

Added IANA_VXLAN_UDP_PORT (4789) definition to vxlan header file so it
can be used by drivers instead of local definition.
Updated drivers which locally defined it as 4789 to use it.

Signed-off-by: Moshe Shemesh <moshe@mellanox.com>
Reviewed-by: Or Gerlitz <ogerlitz@mellanox.com>
Cc: John Hurley <john.hurley@netronome.com>
Cc: Jakub Kicinski <jakub.kicinski@netronome.com>
Cc: Yunsheng Lin <linyunsheng@huawei.com>
Cc: Peng Li <lipeng321@huawei.com>
Reviewed-by: Tariq Toukan <tariqt@mellanox.com>
Acked-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>