nvme: add support for streams and directives

nvme: add support for streams and directives

This adds support for Directives in NVMe, particular for the Streams
directive. Support for Directives is a new feature in NVMe 1.3. It
allows a user to pass in information about where to store the data, so
that it the device can do so most effiently. If an application is
managing and writing data with different life times, mixing differently
retentioned data onto the same locations on flash can cause write
amplification to grow. This, in turn, will reduce performance and life
time of the device.

We default to allocating 4 streams, controller wide, so we can use them
on all name spaces. This is configurable with the 'streams' module
parameter. If a write stream is set in a write, flag is as such before
sending it to the device.

Signed-off-by: Jens Axboe <axboe@kernel.dk>