Support background fsck_ffs(8) on filesystems using journaled soft updates

An earlier addition of code to fsck_ffs(8) allowed it to support
snapshots when running with journalled soft updates. Further
functionality has now been added to fsck_ffs(8) to allow it to use
snapshots to run in background on live filesystems running with
journaled soft updates. This commit enables the use of this functionality.

Tested-by: Peter Holm
Sponsored-by: The FreeBSD Foundation
MFC-after: 2 weeks