Add the ability to adjust directory depths to background fsck_ffs(8).

Commit fe5e6e2 improved FFS directory placement when creating new
directories. It is done by keeping track of the depth of directories
in the filesystem and placing those lower in the tree closer together
while spreading out those higher in the tree.

Fsck_ffs(8) checks these depths and if incorrect adjusts them to
their correct value. When running in background fsck_ffs(8) needs
to be able to make an adjustment to the depth. This commit adds
the sysctl to make such an adjustment and adds the code to fsck_ffs(8)
to use the new sysctl.

MFC after: 1 week
Sponsored by: The FreeBSD Foundation