ixgbe: Replace ndo_add/del_vxlan_port with ndo_add/del_udp_enc_port

This change replaces the network device operations for adding or removing a
VXLAN port with operations that are more generically defined to be used for
any UDP offload port but provide a type.  As such by just adding a line to
verify that the offload type is VXLAN we can maintain the same
functionality.

In addition I updated the socket address family check so that instead of
excluding IPv6 we instead abort of type is not IPv4.  This makes much more
sense as we should only be supporting IPv4 outer addresses on this
hardware.

The last change is that I pulled the rtnl_lock/unlock into the conditional
statement for IXGBE_FLAG2_VXLAN_REREG_NEEDED.  The motivation behind this
is to avoid unneeded bouncing of the mutex which will just slow down the
handling of this call anyway.

Signed-off-by: Alexander Duyck <aduyck@mirantis.com>
Signed-off-by: David S. Miller <davem@davemloft.net>