bp_mapout() deallocates system virtual address space allocated by a previous call to bp_mapin(9F).bp_mapout() should only be called on buffers which have been allocated and are owned by the device driver. It must not be called
on buffers passed to the driver through the strategy(9E)
entry point (for example a filesystem). Because bp_mapin(9F) does not keep a reference
count, bp_mapout() will wipe out any kernel mapping that a layer above the device driver might rely on.
