Merge branch 'gre-seq-collect_md'

William Tu says:

====================
gre: add sequence number for collect md mode.

Currently GRE sequence number can only be used in native tunnel mode.
The first patch adds sequence number support for gre collect
metadata mode, and the second patch tests it using BPF.

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.

A new BFP uapi tunnel flag 'BPF_F_SEQ_NUMBER' is added.
--
v1->v2:
  rename BPF_F_GRE_SEQ to BPF_F_SEQ_NUMBER suggested by Daniel
====================

Signed-off-by: David S. Miller <davem@davemloft.net>