Block layer discard requests

It's nice to see a primitive like this making it into mainline Linux.  We did something
similar for the Itsy handheld back around 2000-2001.  Itsy had flash on the motherboard with
its own drivers, so we didn't need to wait for anything to be added to the SCSI or ATA
interfaces before the idea could be used.  I don't think reordering of discards vs. writes was
possible with our flash drivers.  See:

http://tim-mann.org/papers/hamburgen01itsy.pdf
http://tim-mann.org/papers/SRC-TN-2001-001.pdf

Even earlier than that, the Petal distributed virtual disk had a discard primitive and the
Frangipani filesystem built on top of it used it.  Conventional filesystems could run on Petal
too, but would used disk space more efficiently if modified to use the discard primitive.

http://tim-mann.org/papers/frangipani.pdf
http://portal.acm.org/citation.cfm?id=237157

Basically, this primitive is good for most kinds of virtual disk, to allow savings of one kind
or another if the virtual disk is informed that it can throw some data away.  I was always
kind of surprised it didn't make it into standard OSes long ago.