rcu: add rcu_read_lock_dont_migrate()

migrate_disable() is called to disable migration in the kernel, and it is
often used together with rcu_read_lock().

However, with PREEMPT_RCU disabled, it's unnecessary, as rcu_read_lock()
will always disable preemption, which will also disable migration.

Introduce rcu_read_lock_dont_migrate() and rcu_read_unlock_migrate(),
which will do the migration enable and disable only when PREEMPT_RCU.

Signed-off-by: Menglong Dong <dongml2@chinatelecom.cn>
Reviewed-by: Paul E. McKenney <paulmck@kernel.org>
Link: https://lore.kernel.org/r/20250821090609.42508-2-dongml2@chinatelecom.cn
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

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