Address a performance problem in getnewbuf:

Address a performance problem in getnewbuf:
	In heavy-writing situations, QUEUE_LRU can contain a large number
	of DELWRI buffers at its head.  These buffers must be moved
	to the tail if they cannot be written async in order to reduce
	the scanning time required to skip past these buffers in later
	getnewbuf() calls.

Submitted by:	Matthew Dillon <dillon@apollo.backplane.com>