vxlan: add extack support for create and changelink

This patch adds extack coverage in vxlan link
create and changelink paths. Introduces a new helper
vxlan_nl2flags to consolidate flag attribute validation.

thanks to Johannes Berg for some tips to construct the
generic vxlan flag extack strings.

Signed-off-by: Roopa Prabhu <roopa@cumulusnetworks.com>
Reviewed-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>