net: erspan: refactor existing erspan code

The patch refactors the existing erspan implementation in order
to support erspan version 2, which has additional metadata.  So, in
stead of having one 'struct erspanhdr' holding erspan version 1,
breaks it into 'struct erspan_base_hdr' and 'struct erspan_metadata'.

Signed-off-by: William Tu <u9012063@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>