swiotlb: remove unused field "used" from struct io_tlb_mem

Commit 20347fca ("swiotlb: split up the global swiotlb lock") moved
the number of used slots to struct io_tlb_area, but it did not remove
the field from struct io_tlb_mem.

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