IB/fmr_pool: Stop ib_fmr threads from contributing to load average

I noticed my machine was at a constant load average of 1. This was
because ib_create_fmr_pool calls kthread_create but does not
immediately wake the thread up.

Change to using kthread_run so we enter ib_fmr_cleanup_thread(), set
TASK_INTERRUPTIBLE, then go to sleep.

Signed-off-by: Roland Dreier <rolandd@cisco.com>