gre: do not keep the GRE header around in collect medata mode

For ipgre interface in collect metadata mode, it doesn't make sense for the
interface to be of ARPHRD_IPGRE type. The outer header of received packets
is not needed, as all the information from it is present in metadata_dst. We
already don't set ipgre_header_ops for collect metadata interfaces, which is
the only consumer of mac_header pointing to the outer IP header.

Just set the interface type to ARPHRD_NONE in collect metadata mode for
ipgre (not gretap, that still correctly stays ARPHRD_ETHER) and reset
mac_header.

Fixes: a64b04d8 ("gre: do not assign header_ops in collect metadata mode")
Fixes: 2e15ea39

 ("ip_gre: Add support to collect tunnel metadata.")
Signed-off-by: Jiri Benc <jbenc@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>