selftests/bpf: Close cgrp fd before calling cleanup_cgroup_environment()

There is error log when htab-mem benchmark completes. The error log
looks as follows:

$ ./bench htab-mem -d1
Setting up benchmark 'htab-mem'...
Benchmark 'htab-mem' started.
......
(cgroup_helpers.c:353: errno: Device or resource busy) umount cgroup2

Fix it by closing cgrp fd before invoking cleanup_cgroup_environment().

Signed-off-by: Hou Tao <houtao1@huawei.com>
Link: https://lore.kernel.org/r/20231219135727.2661527-1-houtao@huaweicloud.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

tools/testing/selftests/bpf/benchs/bench_htab_mem.c

@@ -335,6 +335,7 @@ static void htab_mem_report_final(struct bench_res res[], int res_cnt)
 	       " peak memory usage %7.2lfMiB\n",
 	       loop_mean, loop_stddev, mem_mean, mem_stddev, peak_mem / 1048576.0);
 
+	close(ctx.fd);
 	cleanup_cgroup_environment();
 }