kernel - revamp cluster_read API and improve performance

kernel - revamp cluster_read API and improve performance

* Revamp the API to be much less confusing.  Pass a minimum read ahead
  based on the higher level uio length, and a maximum read ahead
  based on the sequential heuristic.  These combine together to determine
  how much read-ahead to do.

  For example if a program is doing random-reads with 1MB read() requests
  the minreq will take precedence, whereas if the program is doing
  sequential-reads of 8K the maxreq will take precedence.

  The sequential heuristic currently maxes out at 128 * 16384 = 2MB.

* Introduce sysctl vfs.max_readahead instead of hardwiring the maximum
  read-ahead.  This defaults to 2MB which is big enough for just about
  anything.  Generally speaking this value should be larger than
  your stripe width.

  Note that currently the sequential heuristic also maxes out at 2MB so
  you cannot go larger than 2MB.

* Also correct bugs that existed in the old cluster_read(), dramatically
  improving performance on striped volumes with large chunk sizes (128K+)
  and a large number of drives (3+).

* No change here but note that HAMMER will currently issue a minimum of
  64K worth of read-ahead when accessing meta-data.