btrfs: rename btrfs_block_group_cache

The type name is misleading, a single entry is named 'cache' while this
normally means a collection of objects. Rename that everywhere. Also the
identifier was quite long, making function prototypes harder to format.

Suggested-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
diff --git a/fs/btrfs/qgroup.c b/fs/btrfs/qgroup.c
index f414fd9..93aeb2e 100644
--- a/fs/btrfs/qgroup.c
+++ b/fs/btrfs/qgroup.c
@@ -3823,7 +3823,7 @@ void btrfs_qgroup_clean_swapped_blocks(struct btrfs_root *root)
  */
 int btrfs_qgroup_add_swapped_blocks(struct btrfs_trans_handle *trans,
 		struct btrfs_root *subvol_root,
-		struct btrfs_block_group_cache *bg,
+		struct btrfs_block_group *bg,
 		struct extent_buffer *subvol_parent, int subvol_slot,
 		struct extent_buffer *reloc_parent, int reloc_slot,
 		u64 last_snapshot)