block: kill ISA_DMA_THRESHOLD usage

block uses ISA_DMA_THRESHOLD for BLK_BOUNCE_ISA. Only SCSI uses
ISA_DMA_THRESHOLD for ancient drivers with non-zero
unchecked_isa_dma. Nowadays drivers (and subsystems) use dma_mask
properly instead of ISA_DMA_THRESHOLD.

Documentation/scsi/scsi_mid_low_api.txt says:

unchecked_isa_dma - 1=>only use bottom 16 MB of ram (ISA DMA addressing
                   restriction), 0=>can use full 32 bit (or better) DMA
                   address space

So block simply uses DMA_BIT_MASK(24) for BLK_BOUNCE_ISA for SCSI.
Signed-off-by: NFUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Acked-by: NJames Bottomley <James.Bottomley@suse.de>
Signed-off-by: NJens Axboe <jaxboe@fusionio.com>

include/linux/blkdev.h

@@ -712,7 +712,7 @@ extern unsigned long blk_max_low_pfn, blk_max_pfn;
 #define BLK_BOUNCE_HIGH		-1ULL
 #endif
 #define BLK_BOUNCE_ANY		(-1ULL)
-#define BLK_BOUNCE_ISA		(ISA_DMA_THRESHOLD)
+#define BLK_BOUNCE_ISA		(DMA_BIT_MASK(24))
 
 /*
  * default timeout for SG_IO if none specified