Fix the fault-in window size logic:
* Use a maximum window size of 1 MiB instead of BIO_MAX_VECS * PAGE_SIZE.
  The previous window size was always one page because the pages variable
  was accidentally being defined and then redefined in
  should_fault_in_pages().
* The nr_dirtied heuristic for guessing when there might be memory
  pressure often results in very small window sizes.  Don't let
  nr_dirtied drop below 8 pages (as btrfs does).
* Compute the window size in units of bytes, not pages.
* Account for page overlap (unaligned iterators).
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>