Convert lists for bufs attached to vnodes from a LIST to a TAILQ.

- Use TAILQ_* macros extensively instead of internal names
- use b_xflags instead of the NOLIST magic number hack in the next pointer
- clean bufs are inserted at the tail rather than the head.
- redo dirty buffer insert so that metadata (negative lbn) goes to the
  tail directly rather than at the HEAD.  This makes a difference when
  inserting dirty data blocks in lbn sorted order since data block
  insertion will not have to bypass all the metadata cruft.  data is
  lbn sorted since it makes sense for clustering and writeback ordering,
  while metadata sorting doesn't help much since the lbn's are
  meaningless when walking the list for writebacks.

Small systems will not notice much (if any) benefit from this, but really
busy systems with large dirty block lists should get a lot more.

I've tested this with softdep, and it doesn't seem to mind the change of
queueing of metadata.

Reviewed (in princible) by: dg
Obtained from: partly from John Dyson's work-in-progress patches in June.

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