perf: reduce packet buffer slicing churn (#2381)

* bench: add packet bytes extraction Criterion benchmark

Adds a Criterion benchmark under easytier/benches/ covering
ZCPacket::payload_bytes and tunnel_payload_bytes at 1280/4096-byte payload
sizes, using iter_batched so ZCPacket construction stays in the setup phase
and is excluded from the timed region.

- Register the [[bench]] entry in easytier/Cargo.toml.
- Document the bench and PACKET_BYTES_* env vars in benches/README.md.

* perf: reduce packet buffer slicing churn

Replace BytesMut::split_off with Buf::advance in ZCPacket bytes
extraction paths (payload_bytes, tunnel_payload_bytes, convert_type,
drop_foreign_header) and in TunZCPacketToBytes, and simplify the
copy_from_slice in new_from_payload.

When the buffer is in its unique (VEC) representation, split_off promotes
it to the shared (ARC) representation, allocating a Shared control block
and bumping the refcount on every call, and pins the buffer in shared
mode. advance only mutates the in-place ptr/len/cap fields, avoiding that
allocation/refcount churn on the TX hot path. The byte data itself is not
copied by either path.

easytier/src/instance/virtual_nic.rs

@@ -24,7 +24,7 @@ use crate::{
 };
 
 use byteorder::WriteBytesExt as _;
-use bytes::{BufMut, BytesMut};
+use bytes::{Buf, BufMut, BytesMut};
 use cidr::{Ipv4Inet, Ipv6Inet};
 use futures::{SinkExt, Stream, StreamExt, lock::BiLock, ready};
 use pin_project_lite::pin_project;
@@ -180,12 +180,13 @@ impl ZCPacketToBytes for TunZCPacketToBytes {
         assert!(payload_offset >= 4);
 
         let ret = if self.has_packet_info {
-            let mut inner = inner.split_off(payload_offset - 4);
+            inner.advance(payload_offset - 4);
             let proto = infer_proto(&inner[4..]);
             self.fill_packet_info(&mut inner[0..4], proto)?;
             inner
         } else {
-            inner.split_off(payload_offset)
+            inner.advance(payload_offset);
+            inner
         };
 
         tracing::debug!(?ret, ?payload_offset, "convert zc packet to tun packet");