block_util: Avoid reallocation of Vec on push

When using DHAT[1] for analysis the amount heap allocations change from:

dhat: Total:     3,186,536 bytes in 41,452 blocks

 to

dhat: Total:     1,059,816 bytes in 34,747 blocks

When running against virtio-block; this still more allocations than
virtio-pmem but a significant improvement without any cost.

[1] https://docs.rs/dhat/latest/dhat/

Signed-off-by: Rob Bradford <robert.bradford@intel.com>

block_util/src/raw_async.rs

@@ -156,9 +156,8 @@ impl AsyncIo for RawFileAsync {
     }
 
     fn complete(&mut self) -> Vec<(u64, i32)> {
-        let mut completion_list = Vec::new();
-
         let cq = self.io_uring.completion();
+        let mut completion_list = Vec::with_capacity(cq.len());
         for cq_entry in cq {
             completion_list.push((cq_entry.user_data(), cq_entry.result()));
         }