[Drbd-dev] remove REQ_OP_WRITE_SAME

[Christoph Hellwig]

Now that we are using REQ_OP_WRITE_ZEROES for all zeroing needs in the
kernel there is very little use left for REQ_OP_WRITE_SAME.  We only
have two callers left, and both just export optional protocol features
to remote systems: DRBD and the target code.

For the target code the only real use case was zeroing offload, which
is kept with this series, and for DRBD I suspect the same based on the
usage.

    git://git.infradead.org/users/hch/block.git delete-write-same

Gitweb:

    http://git.infradead.org/users/hch/block.git/shortlog/refs/heads/delete-write-same

Changes from RFC:
 - add zeroing offload for the SCSI target.