 - In some cases bp->b_vp can be NULL in brelse, don't try to lock the    interlock in that case.
 - In some cases bp->b_vp can be NULL in brelse, don't try to lock the
   interlock in that case.

Found by:	alc

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