[bgp]: Don't flush link-local addr during teardown (#11779)

What is the motivation for this PR?
During the teardown for bgp/test_bgp_dual_asn.py::test_bgp_dual_asn_v4, the PTF interface used for the test is flushed of all IPv4 and IPv6 addresses, including the IPv6 link local address. As a result, NDP and other IPv6 functionality is broken for the flushed interface.

How did you do it?
Specify 'global' scope when flushing the PTF interface to avoid removing IPv6 link local addresses.

How did you verify/test it?
Run the test, verify that all PTF addresses still have an IPv6 link local address after the test is finished.

Author: theasianpianist

tests/bgp/test_bgp_dual_asn.py

@@ -282,7 +282,7 @@ def dual_asn_teardown(self, duthosts, rand_one_dut_hostname, ptfhost):
         logger.info("exabgp stopped")
 
         for port in self.ptf_ports:
-            ptfhost.shell("ip addr flush dev %s" % port)
+            ptfhost.shell("ip addr flush dev {} scope global".format(port))
         duthost.command("sonic-clear arp")
         duthost.command("sonic-clear ndp")
         duthost.command("sonic-clear fdb all")