ip_tunnel: be careful when accessing the inner header

Cong noted that we need the same checks introduced by commit 76c0ddd8
("ip6_tunnel: be careful when accessing the inner header")
even for ipv4 tunnels.

Fixes: c5441932 ("GRE: Refactor GRE tunneling code.")
Suggested-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>