intel_th: msu: Handle kstrndup() failure

Currently, the nr_pages attribute store does not check if kstrndup()
succeeded. Fix this.

Reported-by: Alan Cox <alan.cox@intel.com>
Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Reviewed-by: Laurent Fert <laurent.fert@intel.com>