Introduce stf_mtx to protect global softc list in if_stf.  Add stf_destroy() to handle the common softc destruction path for the two destruction sources: interface cloning destroy, and module unload.
Introduce stf_mtx to protect global softc list in if_stf.  Add
stf_destroy() to handle the common softc destruction path for the
two destruction sources: interface cloning destroy, and module
unload.

NOTE: sc_ro, the cached route for stf conversion, is not synchronized
against concurrent access in this change, that will follow in a future
change.

Reviewed by:	pjd

jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase