ofproto-dpif: Disable miss handling in rule_get_stats().

ofproto-dpif: Disable miss handling in rule_get_stats().

rule_get_stats() is often called when iterating over every rule in
the flow table.  To ensure up-to-date statistics, rule_get_stats()
calls push_all_stats() which can cause flow misses to be handled.
When using the learn action, this can cause rules to be added (and
potentially removed) from the OpenFlow table.  This could corrupt
the caller's data structures, leading to a segmentation fault.
This patch fixes the issue by disabling flow miss handling from
within rule_get_stats().

Bug #15999.
Signed-off-by: Ethan Jackson <ethan@nicira.com>