vxlan_core: move common declarations to private header file

This patch moves common structures and global declarations
to a shared private headerfile vxlan_private.h. Subsequent
patches use this header file as a common header file for
additional shared declarations.

Signed-off-by: Roopa Prabhu <roopa@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>