bcachefs: check the invalid parameter for perf test

The perf_test does not check the number of iterations and threads
when it is zero. If nr_thread is 0, the perf test will keep
waiting for wakekup. If iteration is 0, it will cause exception
of division by zero. This can be reproduced by:
  echo "rand_insert 0 1" > /sys/fs/bcachefs/${uuid}/perf_test
or
  echo "rand_insert 1 0" > /sys/fs/bcachefs/${uuid}/perf_test

Fixes: 1c6fdbd ("bcachefs: Initial commit")
Signed-off-by: Hongbo Li <lihongbo22@huawei.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

Author: Hongbo Li

fs/bcachefs/tests.c

@@ -809,6 +809,11 @@ int bch2_btree_perf_test(struct bch_fs *c, const char *testname,
 	unsigned i;
 	u64 time;
 
+	if (nr == 0 || nr_threads == 0) {
+		pr_err("nr of iterations or threads is not allowed to be 0");
+		return -EINVAL;
+	}
+
 	atomic_set(&j.ready, nr_threads);
 	init_waitqueue_head(&j.ready_wait);