ofproto-dpif-upcall: Don't purge ukeys while in a quiescent state.

ofproto-dpif-upcall: Don't purge ukeys while in a quiescent state.

revalidator_purge() iterates and modifies umap->cmap. This should
not happen in quiescent state, because cmap implementation based
on rcu protected variables. Let's narrow the quiescent period
to avoid possible wrong memory accesses.

CC: Joe Stringer <joe@ovn.org>
Fixes: 9fce0584a643 ("revalidator: Use 'cmap' for storing ukeys.")
Reported-by: Ilya Maximets <i.maximets@samsung.com>
Acked-by: Ilya Maximets <i.maximets@samsung.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>