staging: unisys: visorhba: fail gracefully for thread creation failures

Previously, if visorhba ever failed to create its thread, it would have
subsequently attempted to do a kthread_stop() on an invalid task_struct
pointer.  This was fixed via the new visor_thread_stop() function, which
validates the task_struct pointer before doing the kthread_stop().

Also, the other thread-related fields in visor_thread_info (has_stopped
and id) were not being used, so visor_thead_info was just removed.

We also now spit out an error message if thread creation fails.

Signed-off-by: Tim Sell <Timothy.Sell@unisys.com>
Signed-off-by: David Kershner <david.kershner@unisys.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>