splice: don't pass the address of ->f_pos to methods

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

include/linux/splice.h

@@ -35,6 +35,7 @@ struct splice_desc {
 		void *data;		/* cookie */
 	} u;
 	loff_t pos;			/* file position */
+	loff_t *opos;			/* sendfile: output position */
 	size_t num_spliced;		/* number of bytes already spliced */
 	bool need_wakeup;		/* need to wake up writer */
 };