[JFFS2] Fix calculation of potential summary marker offset on NOR flash.

Helps if we look _inside_ the buffer, rather than adding jeb->offset to
it. Doh.

Signed-off-by: David Woodhouse <dwmw2@infradead.org>
diff --git a/fs/jffs2/scan.c b/fs/jffs2/scan.c
index 42c1ff2..6161808 100644
--- a/fs/jffs2/scan.c
+++ b/fs/jffs2/scan.c
@@ -463,7 +463,7 @@
 	      
 		if (!buf_size) {
 			/* XIP case. Just look, point at the summary if it's there */
-			sm = (void *)buf + jeb->offset - sizeof(*sm);
+			sm = (void *)buf + c->sector_size - sizeof(*sm);
 			if (je32_to_cpu(sm->magic) == JFFS2_SUM_MAGIC) {
 				sumptr = buf + je32_to_cpu(sm->offset);
 				sumlen = c->sector_size - je32_to_cpu(sm->offset);