fix silent write errors (#1358)

fix #1355

some write errors are not reported when calling write_all. That's notably the case of ENOSPC on small buffers (1MiB).
on ext4, the error is catched when calling flush(). This is hopefully the case on most local filesystems, though afaik this assumption doesn't hold for NFS

Reviewed-on: https://git.deuxfleurs.fr/Deuxfleurs/garage/pulls/1358
Co-authored-by: trinity-1686a <trinity@deuxfleurs.fr>
Co-committed-by: trinity-1686a <trinity@deuxfleurs.fr>

src/block/manager.rs

@@ -783,6 +783,7 @@ impl BlockManagerLocked {
 
 		let mut f = fs::File::create(&path_tmp).await?;
 		f.write_all(data).await?;
+		f.flush().await?;
 		mgr.metrics.bytes_written.add(data.len() as u64);
 
 		if mgr.data_fsync {