btrfs: fix space cache inconsistency after error loading it from disk

When loading a free space cache from disk, at __load_free_space_cache(),
if we fail to insert a bitmap entry, we still increment the number of
total bitmaps in the btrfs_free_space_ctl structure, which is incorrect
since we failed to add the bitmap entry. On error we then empty the
cache by calling __btrfs_remove_free_space_cache(), which will result
in getting the total bitmaps counter set to 1.

A failure to load a free space cache is not critical, so if a failure
happens we just rebuild the cache by scanning the extent tree, which
happens at block-group.c:caching_thread(). Yet the failure will result
in having the total bitmaps of the btrfs_free_space_ctl always bigger
by 1 then the number of bitmap entries we have. So fix this by having
the total bitmaps counter be incremented only if we successfully added
the bitmap entry.

Fixes: a67509c ("Btrfs: add a io_ctl struct and helpers for dealing with the space cache")
Reviewed-by: Anand Jain <anand.jain@oracle.com>
CC: stable@vger.kernel.org # 4.4+
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

Author: fdmanana

fs/btrfs/free-space-cache.c

@@ -870,15 +870,16 @@ static int __load_free_space_cache(struct btrfs_root *root, struct inode *inode,
 			}
 			spin_lock(&ctl->tree_lock);
 			ret = link_free_space(ctl, e);
-			ctl->total_bitmaps++;
-			recalculate_thresholds(ctl);
-			spin_unlock(&ctl->tree_lock);
 			if (ret) {
+				spin_unlock(&ctl->tree_lock);
 				btrfs_err(fs_info,
 					"Duplicate entries in free space cache, dumping");
 				kmem_cache_free(btrfs_free_space_cachep, e);
 				goto free_cache;
 			}
+			ctl->total_bitmaps++;
+			recalculate_thresholds(ctl);
+			spin_unlock(&ctl->tree_lock);
 			list_add_tail(&e->list, &bitmaps);
 		}