dm array: fix unreleased btree blocks on closing a faulty array cursor

The cached block pointer in dm_array_cursor might be NULL if it reaches
an unreadable array block, or the array is empty. Therefore,
dm_array_cursor_end() should call dm_btree_cursor_end() unconditionally,
to prevent leaving unreleased btree blocks.

This fix can be verified using the "array_cursor/iterate/empty" test
in dm-unit:
  dm-unit run /pdata/array_cursor/iterate/empty --kernel-dir <KERNEL_DIR>

Signed-off-by: Ming-Hung Tsai <mtsai@redhat.com>
Fixes: fdd1315 ("dm array: introduce cursor api")
Reviewed-by: Joe Thornber <thornber@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>

Author: mingnus

drivers/md/persistent-data/dm-array.c

@@ -960,10 +960,10 @@ EXPORT_SYMBOL_GPL(dm_array_cursor_begin);
 
 void dm_array_cursor_end(struct dm_array_cursor *c)
 {
-	if (c->block) {
+	if (c->block)
 		unlock_ablock(c->info, c->block);
-		dm_btree_cursor_end(&c->cursor);
-	}
+
+	dm_btree_cursor_end(&c->cursor);
 }
 EXPORT_SYMBOL_GPL(dm_array_cursor_end);