support for multi-range discard requests V3

[Sagi Grimberg]

> Hi all,
>
> this series adds support for merging discontiguous discard bios into a
> single request if the driver supports it.  This reduces the number of
> discards sent to the device by about a factor of 5-6 for typical
> workloads on NVMe, and for slower devices that use I/O scheduling
> the number will probably be even bigger but I've not implemented
> that support yet.
>
> Changes since V2:
>   - really fix nr of NVMe ranges to 256
>   - free range on error
>
> Changes since V1:
>   - hardcoded max ranges for NVMe to 255
>   - minor cleanups

Oops, reviewed v1 before noticing this one...