ipf: bail out when ipf state is COMPLETED

ipf: bail out when ipf state is COMPLETED

it is easy to crash ovs when a packet with same id
hits a list that already reassembled completedly
but have not been sent out yet, and this packet is
not duplicate with this hit ipf list due to bigger
offset

    1  0x00007f9fef0ae2d9 in __GI_abort () at abort.c:89
    2  0x0000000000464042 in ipf_list_state_transition at lib/ipf.c:545

Fixes: 4ea96698f667 ("Userspace datapath: Add fragmentation handling.")
Co-authored-by: Wang Li <wangli39@baidu.com>
Signed-off-by: Wang Li <wangli39@baidu.com>
Signed-off-by: Li RongQing <lirongqing@baidu.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>