Fix dead URLs to ftp.kernel.org

URLs to ftp.kernel.org are still exist though the service is closed [0].
This commit fixes the URLs to use www.kernel.org instead.

[0] https://www.kernel.org/shutting-down-ftp-services.html



Signed-off-by: SeongJae Park <sj38.park@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>