net: erspan: create erspan metadata uapi header

The patch adds a new uapi header file, erspan.h, and moves
the 'struct erspan_metadata' from internal erspan.h to it.

Signed-off-by: William Tu <u9012063@gmail.com>
Acked-by: Pravin B Shelar <pshelar@ovn.org>
Signed-off-by: David S. Miller <davem@davemloft.net>