[RFC] Discard update for 3.19

["Martin K. Petersen" <martin.petersen@oracle.com>]

This update is mainly motivated by an attempt to make the
discard_zeroes_data reporting more accurate. As we have discussed
several times in the past we are stuck with pretty weak guarantees from
the T10/T13 standards. And as a result we feel compelled to tighten up
the scenarios under which we advertise discard_zeroes_data since several
applications and subsystems depend on it being accurate.

The first patch is the most controversial. It disables
discard_zeroes_data for libata devices unless they explicitly have been
whitelisted. I had hoped to have a more comprehensive list of drives but
I didn't have much luck in procuring the identify strings that would
allow me to generate the whitelist matching patterns. I could use some
help here.

The second patch tweaks the SCSI disk driver to prefer WRITE SAME w/
UNMAP instead of the UNMAP command since the former has deterministic
behavior.

The lack of a hard discard_zeroes_data guarantees has also prevented us
from having a variant of blkdev_issue_zeroout() that discards if
possible. The last patch in this series will add such a call that the
filesystems and virt block drivers can use to clear and deprovision
block ranges.

-- 
Martin K. Petersen	Oracle Linux Engineering