fs: add RWF_DSYNC aand RWF_SYNC

This is the per-I/O equivalent of O_DSYNC and O_SYNC, and very useful for
all kinds of file servers and storage targets.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/fs/read_write.c b/fs/read_write.c
index cf377cf..3729d8d 100644
--- a/fs/read_write.c
+++ b/fs/read_write.c
@@ -698,12 +698,16 @@
 	struct kiocb kiocb;
 	ssize_t ret;
 
-	if (flags & ~RWF_HIPRI)
+	if (flags & ~(RWF_HIPRI | RWF_DSYNC | RWF_SYNC))
 		return -EOPNOTSUPP;
 
 	init_sync_kiocb(&kiocb, filp);
 	if (flags & RWF_HIPRI)
 		kiocb.ki_flags |= IOCB_HIPRI;
+	if (flags & RWF_DSYNC)
+		kiocb.ki_flags |= IOCB_DSYNC;
+	if (flags & RWF_SYNC)
+		kiocb.ki_flags |= (IOCB_DSYNC | IOCB_SYNC);
 	kiocb.ki_pos = *ppos;
 
 	ret = fn(&kiocb, iter);