do not call markvoldirty() until the mount has been flagged as read-write.
Due to the nature of the msdosfs code, this bug only seemed to appear for
FAT-16 and FAT-32.
This fixes the testcase:
#!/bin/sh
dd if=/dev/zero bs=1m count=1 oseek=119 of=image.msdos
mdconfig -a -t vnode -f image.msdos
newfs_msdos -F 16 /dev/md0 fd120m
mount_msdosfs -o ro /dev/md0 /mnt
mount | grep md0
mount -u -o rw /dev/md0; echo $?
mount | grep md0
umount /mnt
mdconfig -d -u 0
PR: 105412
Tested by: Eugene Grosbein <eugen grosbein pp ru>
a533f9bcf6