Fix a bug with dirty file system handling.

r356313 broke handling of dirty file system because we have restricted
the correction of "odd" byte sequences to checkfat(), and as a result
the dirty bit is never cleared. The old fsck_msdosfs code would write
FAT twice to fix the dirty bit, which is also not ideal.

Fix this by introducing a new rountine, cleardirty() which will perform
the set of clean bit only, and use it in checkfilesys() if we thought
the file system was dirty.

Reviewed by: cem, emaste
MFC after: 3 day
Differential Revision: https://reviews.freebsd.org/D24581