Buffer for force param is deallocated after initialization, so trying
to read it via sysfs results in oops.  Don't allow read access to the
param node.

Spotted by Eric Sesterhenn.
Signed-off-by: NTejun Heo <htejun@gmail.com>
Cc: Eric Sesterhenn <snakebyte@gmx.de>
Signed-off-by: NJeff Garzik <jeff@garzik.org>