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>
SYSCALL_DEFINE1(pipe, int __user *, fildes)
{
	return sys_pipe2(fildes, 0);
}

SYSCALL_DEFINE1(pipe, int __user *, fildes)
{
	return sys_pipe2(fildes, 0);
}
