Disable userfautlfd GC if minor-fault feature isn't available

To ensure reliable experiments we should run the GC only in one mode.

Bug: 160737021
Test: manual
Change-Id: I9a247c0a1a633eec82a36b3704ebdd48a19ba6b3
diff --git a/runtime/gc/collector/mark_compact.cc b/runtime/gc/collector/mark_compact.cc
index 7658440..ecd2b44 100644
--- a/runtime/gc/collector/mark_compact.cc
+++ b/runtime/gc/collector/mark_compact.cc
@@ -114,7 +114,8 @@
       CHECK_EQ(ioctl(fd, UFFDIO_API, &api), 0) << "ioctl_userfaultfd : API:" << strerror(errno);
       gUffdFeatures = api.features;
       close(fd);
-      return true;
+      // Allow this GC to be used only if minor-fault feature is available.
+      return api.features & UFFD_FEATURE_MINOR_SHMEM;
     }
   }
 #endif