ipv6: xfrm: Handle errors reported by xfrm6_find_1stfragopt()

ipv6: xfrm: Handle errors reported by xfrm6_find_1stfragopt()

xfrm6_find_1stfragopt() may now return an error code and we must
not treat it as a length.

Fixes: 2423496af35d ("ipv6: Prevent overrun when parsing v6 header options")
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Acked-by: Craig Gallek <kraig@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
(cherry picked from commit 6e80ac5cc992ab6256c3dae87f7e57db15e1a58c)

Orabug: 26175248
CVE-2017-9074

Signed-off-by: Kirtikar Kashyap <kirtikar.kashyap@oracle.com>
Reviewed-by: Qing Huang <qing.huang@oracle.com>
Reviewed-by: Jack Vogel <jack.vogel@oracle.com>