fuse: document fuse_fsync_in.fsync_flags

The FUSE_FSYNC_DATASYNC flag was introduced by commit b6aeadeda2
("[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>

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 */