btrfs: remove BUG_ON(ret) in alloc_reserved_tree_block

Switch this to an ASSERT() and return the error in the normal case.
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/extent-tree.c

@@ -4761,9 +4761,10 @@ static int alloc_reserved_tree_block(struct btrfs_trans_handle *trans,
 	ret = btrfs_update_block_group(trans, extent_key.objectid,
 				       fs_info->nodesize, true);
 	if (ret) { /* -ENOENT, logic error */
+		ASSERT(!ret);
 		btrfs_err(fs_info, "update block group failed for %llu %llu",
 			extent_key.objectid, extent_key.offset);
-		BUG();
+		return ret;
 	}
 
 	trace_btrfs_reserved_extent_alloc(fs_info, extent_key.objectid,