Merge branch 'gre-add-collect_md-mode-for-ERSPAN-tunnel'

William Tu says:

====================
gre: add collect_md mode for ERSPAN tunnel

This patch series provide collect_md mode for ERSPAN tunnel.  The fist patch
refactors the existing gre_fb_xmit function by exacting the route cache
portion into a new function called prepare_fb_xmit.  The second patch
introduces the collect_md mode for ERSPAN tunnel, by calling the
prepare_fb_xmit function and adding ERSPAN specific logic.  The final patch
adds the test case using bpf_skb_{set,get}_tunnel_{key,opt}.
====================

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