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: 9fd1ff5d ("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>