Call kthread_create() correctly with fixed arguments.

The kernel's kthread_create() function is defined as "..." and there is
no va_list variant at the moment.  The task name is pre-formatted into
a local buffer and passed to kthread_create() with fixed arguments.

Signed-off-by: Chunwei Chen <tuxoko@gmail.com>
Signed-off-by: Tim Chase <tim@chase2k.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #347