bpf: test_bpf: add init_net to dev for flow_dissector

Latest changes in __skb_flow_dissect() assume skb->dev has valid nd_net.
However, this is not true for test_bpf. As a result, test_bpf.ko crashes
the system with the following stack trace:

[ 1133.716622] BUG: unable to handle kernel paging request at 0000000000001030
[ 1133.716623] PGD 8000001fbf7ee067
[ 1133.716624] P4D 8000001fbf7ee067
[ 1133.716624] PUD 1f6c1cf067
[ 1133.716625] PMD 0
[ 1133.716628] Oops: 0000 [#1] SMP PTI
[ 1133.716630] CPU: 7 PID: 40473 Comm: modprobe Kdump: loaded Not tainted 4.19.0-rc5-00805-gca11cc92ccd2 #1167
[ 1133.716631] Hardware name: Wiwynn Leopard-Orv2/Leopard-DDR BW, BIOS LBM12.5 12/06/2017
[ 1133.716638] RIP: 0010:__skb_flow_dissect+0x83/0x1680
[ 1133.716639] Code: 04 00 00 41 0f b7 44 24 04 48 85 db 4d 8d 14 07 0f 84 01 02 00 00 48 8b 43 10 48 85 c0 0f 84 e5 01 00 00 48 8b 80 a8 04 00 00 <48> 8b 90 30 10 00 00 48 85 d2 0f 84 dd 01 00 00 31 c0 b9 05 00 00
[ 1133.716640] RSP: 0018:ffffc900303c7a80 EFLAGS: 00010282
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