spdx: The BSD-2-Clause-FreeBSD identifier is obsolete, drop -FreeBSD

The SPDX folks have obsoleted the BSD-2-Clause-FreeBSD identifier. Catch
up to that fact and revert to their recommended match of BSD-2-Clause.

Discussed with: pfg
MFC After: 3 days
Sponsored by: Netflix

stress2: Added a temporary fix for the UFS+SU "out of inodes" issue

stress2: A unmount of a unionfs file system may return EBUSY.

stress2: Added more unionfs tests