kernel - Fix buffer cache deadlock due to multiple buffer cache sizes
* The buffer daemon was triggering off of dirtybufspace[hw], comparing
it against bufspace. However, bufspace is an aggregation of the
total buffer kva reserved, not the dirty count based on b_bufsize.
* Add dirtykvasize to track the dirty space in terms of the kva reservation
and change the buffer daemons to use that instead.
* systat and other displays still use the bufsize, which is a more
accurate indicator for human consumption.