ofproto: Centralize action checking, doing it at decode time.

ofproto: Centralize action checking, doing it at decode time.

Jarno pointed out that modify_flows__() didn't really need to check every
instance of the flow separately.  After some further investigation I
decided that this was even more of an improvement.

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