mkmux: avoid spiky writes and waste of page cache space

the write of dvr data can become very spiky in some conditions
and this can lead to EOVERFLOW errors on the tuner.
Additionally all data written to disk is put into the linux
page cache and needs to be removed periodically. As we likely
do not need this data very soon avoid keeping it in the
page cache.

Signed-off-by: Peter Lieven <pl@kamp.de>