net: erspan: fix erspan config overwrite

When an erspan tunnel device receives an erpsan packet with different
tunnel metadata (ex: version, index, hwid, direction), existing code
overwrites the tunnel device's erspan configuration with the received
packet's metadata.  The patch fixes it.

Fixes: 1a66a836 ("gre: add collect_md mode to ERSPAN tunnel")
Fixes: f551c91d ("net: erspan: introduce erspan v2 for ip_gre")
Fixes: ef7baf5e ("ip6_gre: add ip6 erspan collect_md mode")
Fixes: 94d7d8f2 ("ip6_gre: add erspan v2 support")
Signed-off-by: William Tu <u9012063@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>