Delete snapshot after opening it when running fsck_ffs(9) in background.

When fsck_ffs(8) runs in background, it creates a snapshot named
fsck_snapshot in the filesystem's .snap directory. The fsck_snapshot
file was removed when the background fsck finished. If the system
crashed or the fsck exited unexpectedly, the fsck_snapshot file
would remain. The snapshot would consume ever more space as the
filesystem changed over time until it was removed by a system
administrator or a future run of background fsck removed it to
create a new snapshot file.

This commit unlinks the .snap/fsck_snapshot file immediately after
opening it so that it will be reclaimed when fsck closes it at the
conclusion of its run. After a system crash, it will be removed as
part of the filesystem cleanup because of its zero reference count.
As only a few milliseconds pass between its creation and unlinking,
there is far less opportunity for it to be accidentally left behind.

PR: 106107
MFC-after: 1 week