io_uring: enable passing a per-io write stream

io_uring: enable passing a per-io write stream

Allow userspace to pass a per-I/O write stream in the SQE:

	__u16 write_stream;

Application can query the supported values from the statx
max_write_streams field.  Unsupported values are ignored by
file operations that do not support write streams or rejected
with an error by those that support them.

Signed-off-by: Kanchan Joshi <joshi.k@samsung.com>
Signed-off-by: Nitesh Shetty <nj.shetty@samsung.com>
Signed-off-by: Keith Busch <kbusch@kernel.org>
[hch: s/write_hints/write_streams/g]
Signed-off-by: Christoph Hellwig <hch@lst.de>