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.

Reviewed by: cem, lidl
Approved by: rstone (mentor)
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D12591