As ftp.kernel.org is closed [0], this commit fixes dead URLs in
documents to use www.kernel.org instead.

[0] https://www.kernel.org/shutting-down-ftp-services.htmlSigned-off-by: NSeongJae Park <sj38.park@gmail.com>
Acked-by: NTheodore Ts'o <tytso@mit.edu>
Acked-by: NDavid S. Miller <davem@davemloft.net>
Reviewed-by: NMauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: NJonathan Corbet <corbet@lwn.net>