block: introduce max_write_streams queue limit

block: introduce max_write_streams queue limit

Drivers with hardware that support write streams need a way to export how
many are available so applications can generically query this.

Note: compared to Keith's origina version this does not automatically
stack the limit.  There is no good way to generically stack them.  For
mirroring or striping just mirroring the write streams will work, but
for everything more complex the stacking drive actually needs to manage
them.

Signed-off-by: Keith Busch <kbusch@kernel.org>
[hch: renamed from max_write_hints to max_write_streams]
Signed-off-by: Christoph Hellwig <hch@lst.de>