vxlan: move fdb code to common location in vxlan_xmit

Handle VXLAN_F_COLLECT_METADATA before VXLAN_F_PROXY. The latter does not
make sense with the former, as it needs populated fdb which does not happen
in metadata mode.

After this cleanup, the fdb code in vxlan_xmit is moved to a common location
and can be later skipped for VXLAN-GPE which does not necessarily carry
inner Ethernet header.

v2: changed commit description to not reference L3 mode

Signed-off-by: Jiri Benc <jbenc@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>