vxlan: combine VXLAN_FLOWBASED into VXLAN_COLLECT_METADATA

IFLA_VXLAN_FLOWBASED is useless without IFLA_VXLAN_COLLECT_METADATA,
so combine them into single IFLA_VXLAN_COLLECT_METADATA flag.
'flowbased' doesn't convey real meaning of the vxlan tunnel mode.
This mode can be used by routing, tc+bpf and ovs.
Only ovs is strictly flow based, so 'collect metadata' is a better
name for this tunnel mode.

Signed-off-by: Alexei Starovoitov <ast@plumgrid.com>
Acked-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>