Tidy up the locking of the bounce pages structures.
Tidy up the locking of the bounce pages structures.

- Use SYSINIT to initialize the structures instead of checking
  total_bpages against 0 in alloc_bounce_pages(), which could lead
  to several initializations being done at the same time.
- Add missing locking in bus_dmamap_load(), the bounce pages mutex
  must be held when calling reserve_bounce_pages() and when touching
  the bounce_map_waitinglist list.
- Remove the useless splhigh() and splx() calls.
- Assert that the bounce pages mutex is held in reserve_bounce_pages()
  to catch regressions.

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