rcu: Latch normal synchronize_rcu() path on flood

rcu: Latch normal synchronize_rcu() path on flood

Currently, rcu_normal_wake_from_gp is only enabled by default
on small systems(<= 16 CPUs) or when a user explicitly set it
enabled.

Introduce an adaptive latching mechanism:
 * Track the number of in-flight synchronize_rcu() requests
   using a new rcu_sr_normal_count counter;

 * If the count reaches/exceeds RCU_SR_NORMAL_LATCH_THR(64),
   it sets the rcu_sr_normal_latched, reverting new requests
   onto the scaled wait_rcu_gp() path;

 * The latch is cleared only when the pending requests are fully
   drained(nr == 0);

 * Enables rcu_normal_wake_from_gp by default for all systems,
   relying on this dynamic throttling instead of static CPU
   limits.

Testing(synthetic flood workload):
  * Kernel version: 6.19.0-rc6
  * Number of CPUs: 1536
  * 60K concurrent synchronize_rcu() calls

Perf(cycles, system-wide):
  total cycles: 932020263832
  rcu_sr_normal_add_req(): 2650282811 cycles(~0.28%)

Perf report excerpt:
  0.01%  0.01%  sync_test/...  [k] rcu_sr_normal_add_req

Measured overhead of rcu_sr_normal_add_req() remained ~0.28%
of total CPU cycles in this synthetic stress test.

Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Tested-by: Samir M <samir@linux.ibm.com>
Suggested-by: Joel Fernandes <joelagnelf@nvidia.com>
Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>