BACKPORT: ipv6: Check ip6_find_1stfragopt() return value properly.

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

BUG=chromium:728984
TEST=Build and run

Change-Id: Ibe276fa2bc5fef002f52fa419f8b68d0a6f7a290
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>
[backport: context changes]
Signed-off-by: Guenter Roeck <groeck@chromium.org>
(cherry picked from commit 7dd7eb9513bd)
Reviewed-on: https://chromium-review.googlesource.com/527373
Reviewed-by: Sonny Rao <sonnyrao@chromium.org>
Reviewed-by: Andrey Ulanov <andreyu@google.com>
(cherry picked from commit 996bbea2b116f79b0f4e128cf3dd4b84a6fc0e5a)
Reviewed-on: https://chromium-review.googlesource.com/530151