vxlan: expose COLLECT_METADATA flag to user space

Two vxlan driver flags FLOWBASED and COLLECT_METADATA need to be set to
make use of its new flow mode. The former already exposed. Expose the latter.

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