dm snapshot: fix chunksize sector conversion

If a snapshot has a smaller chunksize than the page size the
conversion to pages currently returns 0 instead of 1, causing:
kernel BUG in mempool_resize.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Milan Broz <mbroz@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>
Cc: stable@kernel.org

drivers/md/dm-exception-store.c

@@ -131,7 +131,7 @@ struct pstore {
 
 static unsigned sectors_to_pages(unsigned sectors)
 {
-	return sectors / (PAGE_SIZE >> 9);
+	return DIV_ROUND_UP(sectors, PAGE_SIZE >> 9);
 }
 
 static int alloc_area(struct pstore *ps)