Change table chaining layout

Change the page member of the scatterlist structure to be an unsigned
long, and encode more stuff in the lower bits:

- Bits 0 and 1 zero: this is a normal sg entry. Next sg entry is located
  at sg + 1.
- Bit 0 set: this is a chain entry, the next real entry is at ->page_link
  with the two low bits masked off.
- Bit 1 set: this is the final entry in the sg entry. sg_next() will return
  NULL when passed such an entry.

It's thus important that sg table users use the proper accessors to get
and set the page member.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

include/asm-sparc/scatterlist.h

@@ -5,7 +5,7 @@
 #include <linux/types.h>
 
 struct scatterlist {
-	struct page *page;
+	unsigned long page_link;
 	unsigned int offset;
 
 	unsigned int length;