fix: v7 import does not set meta root (ThinkParQ/beegfs-rs#180)

Recent changes skip setting the root inode on the first meta target when importing v7 data.
This made the UPDATE query used to set the meta root silently do nothing since the table is empty.
Instead use an INSERT query to add a single row with the given information.

Author: iamjoemccormick

mgmtd/src/db/import_v7.rs

@@ -313,17 +313,17 @@ fn storage_pools(tx: &Transaction, f: &Path) -> Result<()> {
     Ok(())
 }
 
-/// Sets the root inode info according to the given info
+/// Sets the root inode info according to the given info. Should only be used against a new empty
+/// root_inode table with no rows as part of the import process.
 fn set_meta_root(tx: &Transaction, root_id: NodeId, root_mirrored: bool) -> Result<()> {
-    // overwrite root inode with the correct setting
     if root_mirrored {
         tx.execute(
-            sql!("UPDATE root_inode SET target_id = NULL, group_id = ?1"),
+            sql!("INSERT INTO root_inode (target_id, group_id) VALUES (NULL, ?1)"),
             [root_id],
         )?;
     } else {
         tx.execute(
-            sql!("UPDATE root_inode SET group_id = NULL, target_id = ?1"),
+            sql!("INSERT INTO root_inode (target_id, group_id) VALUES (?1, NULL)"),
             [root_id],
         )?;
     }