ip_gre: add validation for csum_start

Validate csum_start in gre_handle_offloads before we call _gre_xmit so
that we do not crash later when the csum_start value is used in the
lco_csum function call.

This patch deals with ipv4 code.

Fixes: c5441932 ("GRE: Refactor GRE tunneling code.")
Issue: FP3SEC-183
Change-Id: If4c3793676c62aac8b9f3e3027e1452428e2cce0
Reported-by:  <syzbot+ff8e1b9f2f36481e2efc@syzkaller.appspotmail.com>
Signed-off-by: Shreyansh Chouhan <chouhan.shreyansh630@gmail.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
(cherry picked from commit 1d011c4803c72f3907eccfc1ec63caefb852fcbf)