srcu: Dynamically allocate srcu_node array

srcu: Dynamically allocate srcu_node array

This commit shrinks the srcu_struct structure by converting its ->node
field from a fixed-size compile-time array to a pointer to a dynamically
allocated array.  In kernels built with large values of NR_CPUS that boot
on systems with smaller numbers of CPUs, this can save significant memory.

[ paulmck: Apply kernel test robot feedback. ]

Reported-by: A cast of thousands
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>