ofproto-dpif-rid: Use array instead of ofpbuf for recirc_state stack.

ofproto-dpif-rid: Use array instead of ofpbuf for recirc_state stack.

In my opinion, this makes better sense for the stack, because it's not
a packet or a collection of bytes, it's an array of struct mf_subvalue.
(I left it as an ofpbuf for accumulating stack entries during
translation, because the automatic reallocation and especially the stub
support there is helpful.)

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