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This patch adds support for configuring GRE tunnels using the
new rtnl_link interface.  This only works on kernels that have
the new GRE configuration interface.

This is accessed through the "ip link" command.  The previous
tunnel configuration interface "ip tunnel" remains as it is
and should be retained for compatibility with old kernels.
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>