Fix a bug with dirty file system handling.

[delphij]

Summary

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.

Test Plan

patch FAT16 and FAT32 filesystems with clean bit cleared
(modify byte 3 of FAT16 by or'ing 0x70, and modify byte 7 of FAT32 by
or'ing 0x07) and verify that fsck_msdosfs would correctly correct that
byte with and without -p if user answered yes, and don't correct
without -p if user answered no.

[cem]

sbin/fsck_msdosfs/fat.c

664

s/space/memory/ maybe

668–669

Could just use len in place of boot->bpbBytesPerSec. Also for pwrite().

[delphij]

Address reviewer comments.

[delphij]

Please take another look, thanks!