Describe ring log with index,nr

Update mkfs and print to describe the ring blocks with a starting index
and number of blocks instead of a head and tail index.

Signed-off-by: Zach Brown <zab@versity.com>

utils/src/format.h

@@ -316,9 +316,9 @@ struct scoutfs_super_block {
 	__le64 free_blocks;
 	__le64 ring_blkno;
 	__le64 ring_blocks;
-	__le64 ring_head_index;
-	__le64 ring_tail_index;
-	__le64 ring_head_seq;
+	__le64 ring_index;
+	__le64 ring_nr;
+	__le64 ring_seq;
 	__le64 buddy_blocks;
         struct scoutfs_buddy_root buddy_root;
         struct scoutfs_btree_root btree_root;