vxlan: add NETIF_F_FRAGLIST flag for dev features

Some protocol HW GSO requires fraglist supported by the device, like
SCTP. Without NETIF_F_FRAGLIST set in the dev features of vxlan, it
would have to do SW GSO before the packets enter the driver, even
when the vxlan dev and lower dev (like veth) both have the feature
of NETIF_F_GSO_SCTP.

So this patch is to add it for vxlan.

Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>