runit-void: fix issue with fsck on mounted partitions

[classabbyamp]

see #50124

backport of void-linux/void-runit#115

Testing the changes

• I tested the changes in this PR: YES

@dkwo can you test this?

[dkwo]

With bdev_allow_write_mounted=true, the behavior is as before.
Without it, it prints the same message

fsck.ext4: Device or resource busy while trying to open /dev/mapper/cryptroot-root
Filesystem mounted or opened exclusively by another program?

and then it continues (without fsck-ing other partitions like /boot).
So this seems to work as expected in my simple setup.

[dkwo]

I wonder if this fsck checks should rather run in the initramfs, before mounting anything.

[classabbyamp]

with mkinitcpio, fsck is run in the initramfs, idk about dracut