rcu: Eliminate panic when silly boot-time fanout specified

This commit loosens rcutree.rcu_fanout_leaf range checks
and replaces a panic() with a fallback to compile-time values.
This fallback is accompanied by a WARN_ON(), and both occur when the
rcutree.rcu_fanout_leaf value is too small to accommodate the number of
CPUs.  For example, given the current four-level limit for the rcu_node
tree, a system with more than 16 CPUs built with CONFIG_FANOUT=2 must
have rcutree.rcu_fanout_leaf larger than 2.

Reported-by: Peter Zijlstra <[email protected]>
Signed-off-by: Paul E. McKenney <[email protected]>
Reviewed-by: Josh Triplett <[email protected]>

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