Reimplement B_AGE. Have it cycle the buffer in the queue twice instead of

Reimplement B_AGE.  Have it cycle the buffer in the queue twice instead of
placing buffers at the head of the queue (which causes them to be run-down
backwards).  Leave B_AGE set through the write cycle and have the bufdaemon
set the flag when flushing dirty buffers.  B_AGE no longer effects the
ordering of the actual write and is allowed to slide through to the clean
queue when the write completes.