netfilter: gre: Use consistent GRE and PTTP header structure instead of the...

netfilter: gre: Use consistent GRE and PTTP header structure instead of the ones defined by netfilter

There are two existing strutures which defines the GRE and PPTP header.
So use these two structures instead of the ones defined by netfilter to
keep consitent with other codes.

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