btrfs: rename btrfs_space_info_add_old_bytes

This name doesn't really fit with how the space reservation stuff works
now, rename it to btrfs_space_info_free_bytes_may_use so it's clear what
the function is doing.

Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>