netfilter: nft_flow_offload: release dst in case direct xmit path is used

In the Linux kernel, the following vulnerability has been resolved:

netfilter: nft_flow_offload: release dst in case direct xmit path is used

Direct xmit does not use it since it calls dev_queue_xmit() to send
packets, hence it calls dst_release().

kmemleak reports:

unreferenced object 0xffff88814f440900 (size 184):
comm "softirq", pid 0, jiffies 4294951896
hex dump (first 32 bytes):
00 60 5b 04 81 88 ff ff 00 e6 e8 82 ff ff ff ff .`[.............
21 0b 50 82 ff ff ff ff 00 00 00 00 00 00 00 00 !.P.............
backtrace (crc cb2bf5d6):
[<000000003ee17107>] kmem_cache_alloc+0x286/0x340
[<0000000021a5de2c>] dst_alloc+0x43/0xb0
[<00000000f0671159>] rt_dst_alloc+0x2e/0x190
[<00000000fe5092c9>] __mkroute_output+0x244/0x980
[<000000005fb96fb0>] ip_route_output_flow+0xc0/0x160
[<0000000045367433>] nf_ip_route+0xf/0x30
[<0000000085da1d8e>] nf_route+0x2d/0x60
[<00000000d1ecd1cb>] nft_flow_route+0x171/0x6a0 [nft_flow_offload]
[<00000000d9b2fb60>] nft_flow_offload_eval+0x4e8/0x700 [nft_flow_offload]
[<000000009f447dbb>] expr_call_ops_eval+0x53/0x330 [nf_tables]
[<00000000072e1be6>] nft_do_chain+0x17c/0x840 [nf_tables]
[<00000000d0551029>] nft_do_chain_inet+0xa1/0x210 [nf_tables]
[<0000000097c9d5c6>] nf_hook_slow+0x5b/0x160
[<0000000005eccab1>] ip_forward+0x8b6/0x9b0
[<00000000553a269b>] ip_rcv+0x221/0x230
[<00000000412872e5>] __netif_receive_skb_one_core+0xfe/0x110