support for multi-range discard requests

[Christoph Hellwig]

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.