RFC: block: allow write streams on partitions

RFC: block: allow write streams on partitions

By default assign all write streams to partition 1, and add a hack
sysfs files that distributes them all equally.

This is implemented by storing the number of per-partition write
streams in struct block device, as well as the offset to the global
ones, and then remapping the write streams in the I/O submission
path.

The sysfs is hacky and undocumented, better suggestions welcome
from actual users of write stream on partitions.

Signed-off-by: Christoph Hellwig <hch@lst.de>