[Nbd] Proposal to merge WRITE_ZEROES extension into master

[Alex Bligh <alex@...872...>]

I propose we merge the WRITE_ZEROES extension into master. This
effectively promotes the extension to mainline support.

Factors in favour:
* WRITE_ZEROES has been stable since April.
* The branch includes a reference implementation
* Other implementations (gonbdserver and I believe Qemu) use
  it or at least have implementations
* Uncontroversial, minimally impacting
* Reduction in the number of branches

Factors against:
* No kernel implementation to my knowledge, but I don't
  think that's a killer

Wouter and anyone else - any objections / views?

-- 
Alex Bligh