fs: add a write stream field to the kiocb

Prepare for io_uring passthrough of write streams. The write stream
field in the kiocb structure fits into an existing 2-byte hole, so its
size is not changed.

Reviewed-by: Hannes Reinecke <[email protected]>
Reviewed-by: Nitesh Shetty <[email protected]>
Signed-off-by: Christoph Hellwig <[email protected]>
Signed-off-by: Keith Busch <[email protected]>
Signed-off-by: Kanchan Joshi <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jens Axboe <[email protected]>

include/linux/fs.h

@@ -408,6 +408,7 @@ struct kiocb {
 	void			*private;
 	int			ki_flags;
 	u16			ki_ioprio; /* See linux/ioprio.h */
+	u8			ki_write_stream;
 	union {
 		/*
 		 * Only used for async buffered reads, where it denotes the