[Drbd-dev] always use REQ_OP_WRITE_ZEROES for zeroing offload V2

[Christoph Hellwig]

This series makes REQ_OP_WRITE_ZEROES the only zeroing offload
supported by the block layer, and switches existing implementations
of REQ_OP_DISCARD that correctly set discard_zeroes_data to it,
removes incorrect discard_zeroes_data, and also switches WRITE SAME
based zeroing in SCSI to this new method.

The series is against the block for-next tree.

A git tree is also avaiable at:

    git://git.infradead.org/users/hch/block.git discard-rework.2

Gitweb:

    http://git.infradead.org/users/hch/block.git/shortlog/refs/heads/discard-rework.2

Changes since V2:
 - various spelling fixes
 - various reviews captured
 - two new patches from Martin at the end