btrfs: change BUG_ONs to assertions in btrfs_qgroup_trace_subtree()

The only caller do_walk_down() of btrfs_qgroup_trace_subtree() validates
the value of level and uses it several times before it's passed as an
argument. Same for root_eb that's called 'next' in the caller.

Change both BUG_ONs to assertions as this is to assure proper interface
use rather than real errors.
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/qgroup.c

@@ -2505,8 +2505,8 @@ int btrfs_qgroup_trace_subtree(struct btrfs_trans_handle *trans,
 	struct extent_buffer *eb = root_eb;
 	struct btrfs_path *path = NULL;
 
-	BUG_ON(root_level < 0 || root_level >= BTRFS_MAX_LEVEL);
-	BUG_ON(root_eb == NULL);
+	ASSERT(0 <= root_level && root_level < BTRFS_MAX_LEVEL);
+	ASSERT(root_eb != NULL);
 
 	if (!btrfs_qgroup_full_accounting(fs_info))
 		return 0;