sched_ext: Add migration-disabled counter to error state dump

Include the task's migration-disabled counter when dumping task state
during an error exit.

This can help diagnose cases where tasks can get stuck, because they're
unable to migrate elsewhere.

tj: s/nomig/no_mig/ for readability and consistency with other keys.

Signed-off-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Tejun Heo <tj@kernel.org>