codec: optimize buffer reserve for LengthDelimitedCodec::encode (#8333)

The reservation used `n`, the length after `length_adjustment` has been
applied, but the encoder goes on to write `length_field_len` bytes of
header plus `data.len()` bytes of payload. A positive `length_adjustment`
therefore under-reserved by exactly the adjustment on every frame, and a
negative one over-reserved, so the reservation did not match the write in
either of the adjusted configurations shown in the module docs.

tokio-util/src/codec/length_delimited.rs

@@ -631,8 +631,8 @@ impl Encoder<Bytes> for LengthDelimitedCodec {
         })?;
 
         // Reserve capacity in the destination buffer to fit the frame and
-        // length field (plus adjustment).
-        dst.reserve(self.builder.length_field_len + n);
+        // length field.
+        dst.reserve(self.builder.length_field_len + data.len());
 
         if self.builder.length_field_is_big_endian {
             dst.put_uint(n as u64, self.builder.length_field_len);