Currently we allocate a single page to use as intermediate buffer for
holding data that will be used in UFFDIO_COPY. Let's allocate a buffer per
process and make that buffer large enough to hold the largest continuos
chunk.

travis-ci: success for uffd: A new set of improvements
Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>