vxlan: Improve support for header flags

This patch cleans up the header flags of VXLAN in anticipation of
defining some new ones:

- Move header related definitions from vxlan.c to vxlan.h
- Change VXLAN_FLAGS to be VXLAN_HF_VNI (only currently defined flag)
- Move check for unknown flags to after we find vxlan_sock, this
  assumes that some flags may be processed based on tunnel
  configuration
- Add a comment about why the stack treating unknown set flags as an
  error instead of ignoring them

Signed-off-by: Tom Herbert <therbert@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>