improves sigverify discard_excess_packets performance (#22577)

As shown by the added benchmark, current code does worse if there is a
spam address plus a lot of unique addresses.

on current master:
test bench_packet_discard_many_senders  ... bench:   1,997,960 ns/iter (+/- 103,715)
test bench_packet_discard_mixed_senders ... bench:  14,256,116 ns/iter (+/- 534,865)
test bench_packet_discard_single_sender ... bench:   1,306,809 ns/iter (+/- 61,992)

with this commit:
test bench_packet_discard_many_senders  ... bench:   1,644,025 ns/iter (+/- 83,715)
test bench_packet_discard_mixed_senders ... bench:   1,089,789 ns/iter (+/- 86,324)
test bench_packet_discard_single_sender ... bench:     955,234 ns/iter (+/- 55,953)

core/benches/sigverify_stage.rs

@@ -49,11 +49,16 @@ fn run_bench_packet_discard(num_ips: usize, bencher: &mut Bencher) {
 
     bencher.iter(move || {
         SigVerifyStage::discard_excess_packets(&mut batches, 10_000);
+        let mut num_packets = 0;
         for batch in batches.iter_mut() {
             for p in batch.packets.iter_mut() {
+                if !p.meta.discard() {
+                    num_packets += 1;
+                }
                 p.meta.set_discard(false);
             }
         }
+        assert_eq!(num_packets, 10_000);
     });
 }
 
@@ -67,6 +72,43 @@ fn bench_packet_discard_single_sender(bencher: &mut Bencher) {
     run_bench_packet_discard(1, bencher);
 }
 
+#[bench]
+fn bench_packet_discard_mixed_senders(bencher: &mut Bencher) {
+    const SIZE: usize = 30 * 1000;
+    const CHUNK_SIZE: usize = 1024;
+    fn new_rand_addr<R: Rng>(rng: &mut R) -> std::net::IpAddr {
+        let mut addr = [0u16; 8];
+        rng.fill(&mut addr);
+        std::net::IpAddr::from(addr)
+    }
+    let mut rng = thread_rng();
+    let mut batches = to_packet_batches(&vec![test_tx(); SIZE], CHUNK_SIZE);
+    let spam_addr = new_rand_addr(&mut rng);
+    for batch in batches.iter_mut() {
+        for packet in batch.packets.iter_mut() {
+            // One spam address, ~1000 unique addresses.
+            packet.meta.addr = if rng.gen_ratio(1, 30) {
+                new_rand_addr(&mut rng)
+            } else {
+                spam_addr
+            }
+        }
+    }
+    bencher.iter(move || {
+        SigVerifyStage::discard_excess_packets(&mut batches, 10_000);
+        let mut num_packets = 0;
+        for batch in batches.iter_mut() {
+            for packet in batch.packets.iter_mut() {
+                if !packet.meta.discard() {
+                    num_packets += 1;
+                }
+                packet.meta.set_discard(false);
+            }
+        }
+        assert_eq!(num_packets, 10_000);
+    });
+}
+
 #[bench]
 fn bench_sigverify_stage(bencher: &mut Bencher) {
     solana_logger::setup();