[ Upstream commit 7dd7eb95 ]

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

Fixes: 2423496a ("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>
Signed-off-by: Sasha Levin <alexander.levin@verizon.com>