Do not drop vnode interlock in null_checkvp(). null_lock() verifies that v_data is not-null before calling NULLVPTOLOWERVP(), and dropping the interlock allows for reclaim to clean v_data and free the memory.
Do not drop vnode interlock in null_checkvp(). null_lock() verifies that
v_data is not-null before calling NULLVPTOLOWERVP(), and dropping the
interlock allows for reclaim to clean v_data and free the memory.

While there, remove unneeded semicolons and convert the infinite loops
to panics. I have a will to remove null_checkvp() altogether, or leave
it as a trivial stub, but not now.

Reported and tested by:	pho

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