Revert "ip_gre: make ipgre_tunnel_xmit() not parse network header as IP unconditionally"

This reverts commit 412ed947.

The commit is wrong as tiph points to the outer IPv4 header which is
installed at ipgre_header() and not the inner one which is protocol dependant.

This commit broke succesfully opennhrp which use PF_PACKET socket with
ETH_P_NHRP protocol. Additionally ssl_addr is set to the link-layer
IPv4 address. This address is written by ipgre_header() to the skb
earlier, and this is the IPv4 header tiph should point to - regardless
of the inner protocol payload.

Signed-off-by: Timo Teräs <timo.teras@iki.fi>
Signed-off-by: David S. Miller <davem@davemloft.net>