swiotlb: remove unused field "used" from struct io_tlb_mem

Commit 20347fca71a3 ("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 <[e-mail]>
Signed-off-by: Christoph Hellwig <[e-mail]>

include/linux/swiotlb.h

···
  * @nslabs:	The number of IO TLB blocks (in groups of 64) between @start and
  *		@end. For default swiotlb, this is command line adjustable via
  *		setup_io_tlb_npages.
- * @used:	The number of used IO TLB block.
  * @list:	The free list describing the number of free entries available
  *		from each index.
  * @orig_addr:	The original address corresponding to a mapped entry.
···
 	phys_addr_t end;
 	void *vaddr;
 	unsigned long nslabs;
-	unsigned long used;
 	struct dentry *debugfs;
 	bool late_alloc;
 	bool force_bounce;