ipv6: Check ip6_find_1stfragopt() return value properly.

ipv6: Check ip6_find_1stfragopt() return value properly.

Do not use unsigned variables to see if it returns a negative
error or not.

Fixes: 2423496af35d ("ipv6: Prevent overrun when parsing v6 header options")
Reported-by: Julia Lawall <julia.lawall@lip6.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>
(cherry picked from commit 7dd7eb9513bd02184d45f000ab69d78cb1fa1531)

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>
 Conflicts:
	net/ipv6/ip6_offload.c