libnvdimm, btt: cache sector_size in arena_info

In preparation for the error clearing rework, add sector_size in the
arena_info struct.

Signed-off-by: Vishal Verma <vishal.l.verma@intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
diff --git a/drivers/nvdimm/btt.c b/drivers/nvdimm/btt.c
index 15d1b07..9c96530 100644
--- a/drivers/nvdimm/btt.c
+++ b/drivers/nvdimm/btt.c
@@ -566,6 +566,7 @@
 	if (!arena)
 		return NULL;
 	arena->nd_btt = btt->nd_btt;
+	arena->sector_size = btt->sector_size;
 
 	if (!size)
 		return arena;