splice: direct_splice_actor() should not use pos in sd

commit 2cb4b05e upstream.

direct_splice_actor() shouldn't use sd->pos, as sd->pos is for file reading,
file->f_pos should be used instead.

Signed-off-by: Changli Gao <xiaosuo@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>