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>