Previously before you could call unrhdr_delete you needed to
individually free every allocated unit. It is useful to be able to tear
down the unr without having to go through this process, as it is
significantly faster than freeing the individual units.

When unmounting a tmpfs, do not call free_unr.

tmpfs uses unr(9) to allocate inodes. Previously when unmounting it
would individually free the units when it freed each vnode. This is
unnecessary as we can use the newly-added unrhdr_clear function to clear
out the unr in onde go. This measurably reduces the time to unmount a
tmpfs with many files.

This will be committed as two pieces, one for the unr(9) change, one for the
tmpfs usage change.

I tested this using a kld that uses unr, and I also tested it by repeatedly
extracting a ports tree to a tmpfs and umounting that tmpfs. In my testing
the unmount is sped up from ~.31s to ~.24s, on average. Memory does not
appear to be leaking given the output of vmstat -m | grep Unit