net: erspan: fix metadata extraction

Commit d350a823 ("net: erspan: create erspan metadata uapi header")
moves the erspan 'version' in front of the 'struct erspan_md2' for
later extensibility reason.  This breaks the existing erspan metadata
extraction code because the erspan_md2 then has a 4-byte offset
to between the erspan_metadata and erspan_base_hdr.  This patch
fixes it.

Fixes: 1a66a836 ("gre: add collect_md mode to ERSPAN tunnel")
Fixes: ef7baf5e ("ip6_gre: add ip6 erspan collect_md mode")
Fixes: 1d7e2ed2 ("net: erspan: refactor existing erspan code")
Signed-off-by: William Tu <u9012063@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>