It is generally illegal to wait on an unpinned buffer - another CPU could
free it up even before __wait_on_buffer() has taken a ref against the buffer.

Maybe external locking rules will prevent this in specific cases, but that is
really subtle and fragile as locking rules are evolved.

The patch detects people calling wait_on_buffer() against an unpinned buffer
and issues a diagnostic.

Also remove the get_bh() from __wait_on_buffer().  It is too late.