btrfs: do not account twice for inode ref when reserving metadata units

When reserving metadata units for creating an inode, we don't need to
reserve one extra unit for the inode ref item because when creating the
inode, at btrfs_create_new_inode(), we always insert the inode item and
the inode ref item in a single batch (a single btree insert operation,
and both ending up in the same leaf).

As we have accounted already one unit for the inode item, the extra unit
for the inode ref item is superfluous, it only makes us reserve more
metadata than necessary and often adding more reclaim pressure if we are
low on available metadata space.
Reviewed-by: NNikolay Borisov <nborisov@suse.com>
Signed-off-by: NFilipe Manana <fdmanana@suse.com>
Reviewed-by: NDavid Sterba <dsterba@suse.com>
Signed-off-by: NDavid Sterba <dsterba@suse.com>

fs/btrfs/inode.c

@@ -6164,12 +6164,15 @@ int btrfs_new_inode_prepare(struct btrfs_new_inode_args *args,
 		(*trans_num_items)++;
 	} else {
 		/*
-		 * 1 to add inode ref
 		 * 1 to add dir item
 		 * 1 to add dir index
 		 * 1 to update parent inode item
+		 *
+		 * No need for 1 unit for the inode ref item because it is
+		 * inserted in a batch together with the inode item at
+		 * btrfs_create_new_inode().
 		 */
-		*trans_num_items += 4;
+		*trans_num_items += 3;
 	}
 	return 0;
 }