selftests: bridge_mdb: Use MDB get instead of dump

BugLink: https://bugs.launchpad.net/bugs/2061814

[ Upstream commit e8bba9e ]

Test the new MDB get functionality by converting dump and grep to MDB
get.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Acked-by: Nikolay Aleksandrov <razor@blackwall.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
Stable-dep-of: dd6b345 ("selftests: forwarding: Suppress grep warnings")
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Portia Stephens <portia.stephens@canonical.com>
Signed-off-by: Roxana Nicolescu <roxana.nicolescu@canonical.com>