Revert "rcu:move TINY_RCU from softirq to kthread"

This reverts commit 8a7b6c5b4dcc071fe20b1b63c06c777483c4efc1.
Revert "rcu: add priority-inversion testing to rcutorture"

This reverts commit 690b2817f1e31a4f2cd5a8884459c2f4e093f113.