ofproto: Optimize the case of a repeated learn action execution.

When the target flow exists already as intended, only the 'modified'
time needs to be updated.  Allow modification without taking the
'ofproto_mutex' by always using the 'rule->mutex' when accessing the
'modified' time.

Signed-off-by: Jarno Rajahalme <jrajahalme@nicira.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>