f2fs: should fail mount when trying to recover data on read-only dev

If device is read-only, we should not proceed data recovery.
But, if the previous checkpoint was done by normal clean shutdown, it's safe to
proceed the recovery, since there will be no data to be recovered.

Reviewed-by: Chao Yu <chao2.yu@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>