In handle_group_mod() cases where adding a group failed, nothing freed the
list of buckets, causing a leak.  The same was true in every case of
modifying a group.  This commit fixes the problem by changing add_group()
to never steal or free the buckets (modify_group() already acted this way)
and then making handle_group_mod() always free the buckets when it's done.

This approach might at first raise objections, because it makes add_group()
copy the buckets instead of just take the existing ones.  On branch-2.5
and master, there's a good reason for that--please see the original commit
for explanation.  On this backport to branch-2.4, though, we just use this
approach to avoid having to carefully write a new version for the backport.

Found by pain and suffering.

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