Move clear_unrhdr to tmpfs_free_tmp.
Clearing the unr in tmpfs_unmount is not correct. In the case of
multiple references to the tmpfs mount (e.g. when there are lookup
threads using it) it will not be the one to finish tmpfs_free_tmp. In
those cases tmpfs_free_node_locked will be the final one to execute
tmpfs_free_tmp, and until then the unr must be valid.
Reported by: pho
Approved/reviewed by: rstone (mentor)
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D12749