UPSTREAM: udp: initialize is_flist with 0 in udp_gro_receive

Without NAPI_GRO_CB(skb)->is_flist initialized, when the dev doesn't
support NETIF_F_GRO_FRAGLIST, is_flist can still be set and fraglist
will be used in udp_gro_receive().

So fix it by initializing is_flist with 0 in udp_gro_receive.

Fixes: 9fd1ff5d2ac7 ("udp: Support UDP fraglist GRO/GSO.")
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Acked-by: Steffen Klassert <steffen.klassert@secunet.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Bug: 167707933
Change-Id: Icd2977dfb7506ce0f83d88f52184aa282bc2cac3
(cherry picked from commit bde1b56f898ca8112912d7b36e55e1543b3be0cf)
Signed-off-by: Hyunsoon Kim <h10.kim@samsung.com>