runner: check disk limit at dumping kmsg

It was reported that kernel dumps can grow beyond disk limit size
so add checks for it and report error if that happen.

v2: return number of written bytes (Petri)

Reported-by: Karol Krol <karol.krol@intel.com>
Ref: #129


Cc: Petri Latvala <adrinael@adrinael.net>
Cc: Arkadiusz Hiler <arkadiusz.hiler@intel.com>
Cc: Juha-Pekka Heikkila <juhapekka.heikkila@gmail.com>
Signed-off-by: Kamil Konieczny <kamil.konieczny@linux.intel.com>
Reviewed-by: Petri Latvala <adrinael@adrinael.net>
Reviewed-by: Andrzej Hajda <andrzej.hajda@intel.com>