mapping.  Which is good if pages were mapped - but if they were provided
by someone else and just copied then bad things happen - pages are
released once here, and once by caller, leading to user triggerable BUG
at include/linux/mm.h:246.
Signed-off-by: NPetr Vandrovec <petr@vandrovec.name>
Signed-off-by: NJens Axboe <jens.axboe@oracle.com>