swiotlb: remove unused fields in io_tlb_mem

Commit 20347fca

 ("swiotlb: split up the global swiotlb lock") splits
io_tlb_mem into multiple areas. Each area has its own lock and index. The
global ones are not used so remove them.

Signed-off-by: Chao Gao <chao.gao@intel.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>