selftests: mirror_gre_bridge_1q: Avoid changing PVID while interface is operational

selftests: mirror_gre_bridge_1q: Avoid changing PVID while interface is operational

In emulated environments, the bridge ports enslaved to br1 get a carrier
before changing br1's PVID. This means that by the time the PVID is
changed, br1 is already operational and configured with an IPv6
link-local address.

When the test is run with netdevs registered by mlxsw, changing the PVID
is vetoed, as changing the VID associated with an existing L3 interface
is forbidden. This restriction is similar to the 8021q driver's
restriction of changing the VID of an existing interface.

Fix this by taking br1 down and bringing it back up when it is fully
configured.

With this fix, the test reliably passes on top of both the SW and HW
data paths (emulated or not).

Fixes: dbb90fdbbb27 ("selftests: forwarding: Test mirror-to-gretap w/ UL 802.1q")
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Link: https://lore.kernel.org/r/20220502084507.364774-1-idosch@nvidia.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>