exfat: retain 'VolumeFlags' properly

MediaFailure and VolumeDirty should be retained if these are set before
mounting.

In '3.1.13.3 Media Failure Field' of exfat specification describe:

 If, upon mounting a volume, the value of this field is 1,
 implementations which scan the entire volume for media failures and
 record all failures as "bad" clusters in the FAT (or otherwise resolve
 media failures) may clear the value of  this field to 0.

Therefore, We should not clear MediaFailure without scanning volume.

In '8.1 Recommended Write Ordering' of exfat specification describe:

 Clear the value of the VolumeDirty field to 0, if its value prior to
 the first step was 0.

Therefore, We should not clear VolumeDirty after mounting.
Also rename ERR_MEDIUM to MEDIA_FAILURE.

Signed-off-by: Tetsuhiro Kohada <kohada.t2@gmail.com>
Signed-off-by: Namjae Jeon <namjae.jeon@samsung.com>