vxlan: set needed headroom correctly

vxlan_setup is called when allocating the net_device, i.e. way before
vxlan_newlink (or vxlan_dev_configure) is called. This means
vxlan->default_dst is actually unset in vxlan_setup and the condition that
sets needed_headroom always takes the else branch.

Set the needed_headrom at the point when we have the information about
the address family available.

Fixes: e4c7ed41 ("vxlan: add ipv6 support")
Fixes: 2853af6a ("vxlan: use dev->needed_headroom instead of dev->hard_header_len")
CC: Cong Wang <cwang@twopensource.com>
Signed-off-by: Jiri Benc <jbenc@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>