assert_vop_locked: make the assertion race-free and more efficient
assert_vop_locked: make the assertion race-free and more efficient

this is really a minor improvement for the sake of correctness

MFC after:	6 days

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