lazy-pages: add ability to limit background transfer size

Currently, once we get to transfer pages in the "background", we try to
fetch the entire IOV at once. For large IOVs this may impact #PF latency
for the #PF events occurred during the transfer.

Let's add a simple heuristic for controlling size of the background
transfers. Initially, the transfer will be limited to some default value.
Every time we transfer a chunk we increase the transfer size until it
reaches a pre-defined maximal size. A page fault event resets the
background transfer size to its initial value.

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