dm log writes: fix check of kthread_run() return value

commit 91e630d9 upstream.

The kthread_run() function returns either a valid task_struct or
ERR_PTR() value, check for NULL is invalid.  This change fixes potential
for oops, e.g. in OOM situation.

Signed-off-by: Vladimir Zapolskiy <vz@mleia.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>