Improve the logic for find free port in distributed training

1. we should log  the updated free port to inform users since it is different from the expected port
2. what if the port set by users is not free? This PR does not fix such a case, which is also a common case.

_Originally posted by @ZwwWayne in https://github.com/open-mmlab/mmcv/pull/1846#discussion_r846723708_