btrfs: use assertion helpers for spinning writers

Use the helpers where open coded. On non-debug builds, the warnings will
not trigger and extent_buffer::spining_writers become unused and can be
moved to the appropriate section, saving a few bytes.

Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: David Sterba <dsterba@suse.com>