new helper: copy_page_from_iter()

parallel to copy_page_to_iter().  pipe_write() switched to it (and became
->write_iter()).

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
static inline void __pipe_unlock(struct pipe_inode_info *pipe)
{
	mutex_unlock(&pipe->mutex);
}

static inline void __pipe_unlock(struct pipe_inode_info *pipe)
{
	mutex_unlock(&pipe->mutex);
}
