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: 1c6fdbd8 ("bcachefs: Initial commit")
Signed-off-by: Hongbo Li <lihongbo22@huawei.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>