netfilter: nft_inner: set tunnel offset to GRE header offset

Set inner tunnel offset to the GRE header, this is redundant to existing
transport header offset, but this normalizes the handling of the tunnel
header regardless its location in the layering. GRE version 0 is overloaded
with RFCs, the type decorator in the inner expression might also be useful
to interpret matching fields from the netlink delinearize path in userspace.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>