lazy-pages: cache buffer size in the lazy_pages_info

Instead of recalculating required for lazy_pages_info->buf when copying
IOVs at fork() time, keep the size of the buffer in the lazy_pages_info
struct.

Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>