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 per name space, but it is
configurable with the 'streams_per_ns' module option. If a write stream
is set in a write, flag is as such before sending it to the device. The
streams are allocated lazily - if we get a write request with a life
time hint, then background allocate streams and use them once that
is done.

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