[bpf: Fix reference count leak in bpf_prog_test_run_xdp()]

In the Linux kernel, the following vulnerability has been resolved: bpf: Fix reference count leak in bpf_prog_test_run_xdp() syzbot is reporting unregister_netdevice: waiting for sit0 to become free. Usage count = 2 problem. A debug printk() patch found that a refcount is obtained at xdp_convert_md_to_buff() from bpf_prog_test_run_xdp(). According to commit ec94670fcb3b ("bpf: Support specifying ingress via xdp_md context in BPF_PROG_TEST_RUN"), the refcount obtained by xdp_convert_md_to_buff() will be released by xdp_convert_buff_to_md(). Therefore, we can consider that the error handling path introduced by commit 1c1949982524 ("bpf: introduce frags support to bpf_prog_test_run_xdp()") forgot to call xdp_convert_buff_to_md().