Optimize klp_shadow performance with improved locking and RCU usage

[HajimohammadiNet]

This commit enhances the performance of the klp_shadow module by applying several optimizations:

• Minimized spinlock contention: Critical sections using spinlocks are reduced to only necessary regions, while memory allocation and initialization are performed outside the lock. This reduces contention and improves parallelism.

• Branch prediction hints: Added likely() and unlikely() macros for frequently occurring conditions, improving CPU branch prediction and overall performance.

• RCU-safe lookups: Replaced hash_for_each_possible() with hash_for_each_possible_rcu() where appropriate, allowing for more efficient read-side lookups without blocking writes.

• Optimized memory freeing: Improved shadow variable freeing by batching operations under the lock and using hash_for_each_rcu() in klp_shadow_free_all() for faster traversal.

These changes improve concurrency and efficiency in managing shadow variables, especially under high-load scenarios, while maintaining safety through proper RCU mechanisms.

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[MrZodoxVpython]

gorgeous move