lib: Refactor mac-learning updates.

lib: Refactor mac-learning updates.

Make mac table update functions part of the mac-learning module, which
also helps in figuring what is the minimal set of struct flow fields
needed for the update.  Use this to change the xlate cache entry for
XC_NORMAL to not take a copy of the struct flow, but only save the
in_port, dl_src, and some auxiliary fields.  This reduces the memory
burden of XC_NORMAL by roughly 0.5kb.

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