kernel - Fix cluster_read random I/O heuristic

kernel - Fix cluster_read random I/O heuristic

* cluster_read was improperly accounting for the seqcount in the random
  I/O heuristic, failing to reduce the count in the loop which led to
  fairly maximal sequential read-ahead even for random I/O.

  Properly reduce the seqcount for both the initial buffer and in the
  read-ahead loop, which has the effect of terminating the loop early
  or not running it at all when the I/O is random.

* HAMMER was not scaling the seqcount for the heuristic to BKVASIZE.

* Thanks to Jan for spending the time required to bisect and track down the
  problem!

Reported-by: Jan Lentfer <Jan.Lentfer@web.de>