ofproto-dpif: Zero-out subfacet counters when installation fails.

ofproto-dpif: Zero-out subfacet counters when installation fails.

When deleting subfacets, subfacet_uninstall() asserts that the
subfacet's counters are zero to make sure we don't lose counters.  We
have seen cases where a subfacet cannot be installed, but the counters
have values.  This should not happen and indicates a bug, since we
shouldn't create a subfacet if the datapath has a flow.  A buggy
datapath could trigger this, so just zero out the counters and log an
error.

Bug #18460.

Signed-off-by: Justin Pettit <jpettit@nicira.com>