fuse: document fuse_fsync_in.fsync_flags

The FUSE_FSYNC_DATASYNC flag was introduced by commit b6aeaded
("[PATCH] FUSE - file operations") as a magic number.  No new values have
been added to fsync_flags since.
Signed-off-by: Alan Somers <asomers@FreeBSD.org>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fs/fuse/file.c

@@ -464,7 +464,7 @@ int fuse_fsync_common(struct file *file, loff_t start, loff_t end,
 
 	memset(&inarg, 0, sizeof(inarg));
 	inarg.fh = ff->fh;
-	inarg.fsync_flags = datasync ? 1 : 0;
+	inarg.fsync_flags = datasync ? FUSE_FSYNC_FDATASYNC : 0;
 	args.in.h.opcode = opcode;
 	args.in.h.nodeid = get_node_id(inode);
 	args.in.numargs = 1;

include/uapi/linux/fuse.h

@@ -360,6 +360,13 @@ struct fuse_file_lock {
  */
 #define FUSE_POLL_SCHEDULE_NOTIFY (1 << 0)
 
+/**
+ * Fsync flags
+ *
+ * FUSE_FSYNC_FDATASYNC: Sync data only, not metadata
+ */
+#define FUSE_FSYNC_FDATASYNC	(1 << 0)
+
 enum fuse_opcode {
 	FUSE_LOOKUP		= 1,
 	FUSE_FORGET		= 2,  /* no reply */