block-backend: fix blk_co_flush prototype to mention coroutine_fn

We already have this marker for the blk_co_flush function declaration in
block/block-backend.c. Add it in the header too.

Signed-off-by: Vladimir Sementsov-Ogievskiy <[email protected]>
Message-Id: <[email protected]>
Reviewed-by: Eric Blake <[email protected]>
[eblake: wording tweak]
Signed-off-by: Eric Blake <[email protected]>

Author: Vladimir Sementsov-Ogievskiy

include/sysemu/block-backend.h

@@ -183,7 +183,7 @@ BlockAIOCB *blk_aio_ioctl(BlockBackend *blk, unsigned long int req, void *buf,
                           BlockCompletionFunc *cb, void *opaque);
 int coroutine_fn blk_co_pdiscard(BlockBackend *blk, int64_t offset,
                                  int64_t bytes);
-int blk_co_flush(BlockBackend *blk);
+int coroutine_fn blk_co_flush(BlockBackend *blk);
 int blk_flush(BlockBackend *blk);
 int blk_commit_all(void);
 void blk_inc_in_flight(BlockBackend *blk);