librbd: don't remove an image w/ incompatible features

Fixes: http://tracker.ceph.com/issues/18315
Signed-off-by: Dongsheng Yang <dongsheng.yang@easystack.cn>
(cherry picked from commit f76127b)

Conflicts:
	src/librbd/internal.cc - trivial conflict with
            3df756c which is not needed in
            hammer

src/librbd/internal.cc

@@ -1722,6 +1722,9 @@ int validate_pool(IoCtx &io_ctx, CephContext *cct) {
     int r = open_image(ictx);
     if (r < 0) {
       ldout(cct, 2) << "error opening image: " << cpp_strerror(-r) << dendl;
+      if (r != -ENOENT) {
+	return r;
+      }
     } else {
       string header_oid = ictx->header_oid;
       old_format = ictx->old_format;