swiotlb: use the atomic counter of total used slabs if available

If DEBUG_FS is enabled, the cost of keeping an exact number of total
used slabs is already paid. In this case, there is no reason to use an
inexact number for statistics and kernel messages.

Signed-off-by: Petr Tesarik <petr.tesarik.ext@huawei.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>