Revert "rcu:apply TINY_PREEMPT_RCU read-speedup to TREE_PREEMPT_RCU"

This reverts commit 795f6287a232d6cee03f080f82dc88b783af1ac6.
Revert "rcu:combine duplicate code, courtesy of CONFIG_PREEMPT_RCU"

This reverts commit ede9b63e66e1130607c508e8406e55ff12c28058.
Revert "rcu:repair code-duplication FIXMEs"

This reverts commit bac4623e99fbbb89a9bb6256b00f141a859cef0e.
rcu:combine duplicate code, courtesy of CONFIG_PREEMPT_RCU