Fix hopefully all possible deadlocks that can occur when mixed block sizes

Fix hopefully all possible deadlocks that can occur when mixed block sizes
are used with the buffer cache.  The fix is simply to base the limiting
and flushing code on a byte count rather then a buffer count.

This will allow UFS to utilizes a greater number of dirty buffers and
will cause HAMMER to use fewer.  This also makes tuning the buffer cache
a whole lot easier.