bpf: use rcu_read_lock_dont_migrate() for bpf_iter_run_prog()

bpf: use rcu_read_lock_dont_migrate() for bpf_iter_run_prog()

Use rcu_read_lock_dont_migrate() and rcu_read_unlock_migrate() in
bpf_iter_run_prog to obtain better performance when PREEMPT_RCU is
not enabled.

Signed-off-by: Menglong Dong <dongml2@chinatelecom.cn>
Link: https://lore.kernel.org/r/20250821090609.42508-5-dongml2@chinatelecom.cn
Signed-off-by: Alexei Starovoitov <ast@kernel.org>