kernel - Remove most buffer_map contention on 64-bit systems

kernel - Remove most buffer_map contention on 64-bit systems

* Set BKVASIZE to MAXBSIZE (65536) on 64-bit systems.  This has the effect
  of always reserving a maximal amount of KVM for each buffer cache buffer.

* The change removes most buffer_map interactions once the system caches
  have stabilized.

* The change removes the need to defragment the buffer cache.

* Significant performance improvement for HAMMER1 and HAMMER2 which use
  larger buffers and were hitting degenerate fragmentation issues before
  this change.

* But also results in lower buffer data density when buffering data for
  smaller files, so may have a slight detrimental effect on UFS and on
  the amount of time dirty data can be cached before being flushed to
  disk.

* NOTE: The 64K limit is for normal buffers and is unrelated to the
 	physical cluster buffer (pbuf) limit of 128KB (MAXPHYS).