ofproto-dpif: Destroy facets on ofproto removal.

ofproto-dpif: Destroy facets on ofproto removal.

Facets maintain a pointer to their owning ofproto-dpif, and therefore
when an ofproto-dpif is destroyed all of their child facets should be
destroyed along with it.  If they aren't, it's possible a subfacet
looked up in the dpif-backer could access it's parent facet and
therefore a defunct parent ofproto causing a segmentation fault.

Bug #19421.
Bug #19423.
Diagnosed-by: Ben Pfaff <blp@nicira.com>
Signed-off-by: Ethan Jackson <ethan@nicira.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Ethan Jackson <ethan@nicira.com>