vnet: disable the expansion of the heap allocated for classifier tables

vnet: disable the expansion of the heap allocated for classifier tables

Classifier data structures assume the contiguous chunk of memory
within the heap. Default heap flags for dlmalloc allow for heap growth.
When that happens, the memory becomes discontiguous. This results
in symptoms that are more cryptic than necessary.

Disabling the expand makes the session allocation behavior
of the classifier the same for dlmalloc as for the legacy allocator.

Change-Id: I2f725b5f78a31a8eaa5f5a20dfdd7e1129662f6a
Signed-off-by: Andrew Yourtchenko <ayourtch@gmail.com>