lazy-pages: make uffd_io_complete more robust

Make sure we handle various corner cases:
* we received less pages than requested
* the request was capped because of unmap/remap etc
* the process has exited underneath us

Currently we are freeing the request once we've found the address to use
with uffd_copy(). Instead, let's keep the request object around, use it to
properly calculate number of pages we pass to uffd_copy() and then re-add
tailing range (if any) to the IOVs list.

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