simplify disksort, please review.

[Jeff Roberson]

http://www.chesapeake.net/~jroberson/disksort.diff

Our disksort algorithm used to be complicated by the BIO_ORDERED flag,
which could cause us to make some exceptions in the sorting.  When the
ordered support was removed we never simplified the algorithm.  The patch
above gets rid of the switch point and associated logic.  It's now a
simple hinted insertion sort with a one way scan.  Since it's a fairly
central algorithm, I'd appreciate a review.

Cheers,
Jeff