netfilter: gre: Use consistent GRE_* macros instead of ones defined by netfilter.

There are already some GRE_* macros in kernel, so it is unnecessary
to define these macros. And remove some useless macros

Signed-off-by: Gao Feng <fgao@ikuai8.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>