gre: add sequence number for collect md mode.

Currently GRE sequence number can only be used in native
tunnel mode.  This patch adds sequence number support for
gre collect metadata mode.  RFC2890 defines GRE sequence
number to be specific to the traffic flow identified by the
key.  However, this patch does not implement per-key seqno.
The sequence number is shared in the same tunnel device.
That is, different tunnel keys using the same collect_md
tunnel share single sequence number.

Signed-off-by: William Tu <u9012063@gmail.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>