Linux bridge agent should also handle empty before/after notifications properly

[KaiLin]

I am seeing the bug: https://bugs.launchpad.net/neutron/+bug/1367881
And it fixed the bug by handling empty before/after notifications in l2pop code.
Then I find there is the same problem in the method "_fdb_chg_ip" of linuxbridge_neutron_agent.

after = state.get('after')
                for mac, ip in after:
                    self.agent.br_mgr.add_fdb_ip_entry(mac, ip, interface)

 before = state.get('before')
                for mac, ip in before:
                    self.agent.br_mgr.remove_fdb_ip_entry(mac, ip, interface)

I think we should also change it.Otherwise, it may cause some similar problems.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/166931

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/166931
Committed: https://git.openstack.org/cgit/openstack/neutron/commit/?id=cd4d2f03e83c326f849a0b18a666a406087c94e7
Submitter: Jenkins
Branch: master

commit cd4d2f03e83c326f849a0b18a666a406087c94e7
Author: Eugene Nikanorov <email address hidden>
Date: Mon Mar 23 07:51:06 2015 +0400

    Fix handling of before/after notifications in linuxbridge agent

    Avoid problem similar to described in bug #1367881

    Change-Id: I76059469c20be9161743ba730e46da1789ded4a8
    Closes-Bug: #1407887
    Related-Bug: #1367881