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).