ofp-actions: Waste less memory in learn actions.

ofp-actions: Waste less memory in learn actions.

Make the immediate data member 'src_imm' of a learn spec allocated at
the end of the action for just the right size.  This, together with
some structure packing saves on average of ~128 bytes for each learn
spec in each learn action.  Typical learn actions have about 4 specs
each, so this amounts to saving about 0.5kb for each learn action.

Signed-off-by: Jarno Rajahalme <jarno@ovn.org>
Acked-by: Ben Pfaff <blp@ovn.org>