buffer is 4x needed size (openzfs#486)

I noticed that these Vec's default to Vec<u32>, so their size in bytes is 4x their .len(). We only need "len" bytes, so this is allocating and zeroing out 4x the data that we need. Change it to be Vec<u8>

cmd/zfs_object_agent/zettacache/src/block_access.rs

@@ -143,7 +143,7 @@ impl BlockAccess {
         let begin = Instant::now();
         let _permit = self.outstanding_reads.acquire().await.unwrap();
         let vec = tokio::task::spawn_blocking(move || {
-            let mut v = Vec::new();
+            let mut v: Vec<u8> = Vec::new();
             // XXX use unsafe code to avoid double initializing it?
             // XXX directio requires the pointer to be sector-aligned, requiring this grossness
             v.resize(usize::from64(extent.size) + sector_size, 0);
@@ -184,7 +184,7 @@ impl BlockAccess {
         let begin = Instant::now();
         let _permit = self.outstanding_writes.acquire().await.unwrap();
         tokio::task::spawn_blocking(move || {
-            let mut v = Vec::new();
+            let mut v: Vec<u8> = Vec::new();
             // XXX directio requires the pointer to be sector-aligned, requiring this grossness
             v.resize(length + sector_size, 0);
             let aligned = unsafe {