When vtruncbuf() iterates over the vnode buffer list, lock buffer object before checking the validity of the next buffer pointer. Otherwise, the buffer might be reclaimed after the check, causing iteration to run into wrong buffer.
When vtruncbuf() iterates over the vnode buffer list, lock buffer object
before checking the validity of the next buffer pointer. Otherwise, the
buffer might be reclaimed after the check, causing iteration to run into
wrong buffer.

Reported and tested by:	pho
MFC after:	1 week

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