ip6erspan: improve error handling for erspan version number.

When users fill in incorrect erspan version number through
the struct erspan_metadata uapi, current code skips pushing
the erspan header but continue pushing the gre header, which
is incorrect.  The patch fixes it by returning error.

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