link_linux: add support for IFLA_MACVLAN_BC_QUEUE_LEN(_USED)

[champtar]

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Fixes #974

[aboch]

Please add some UT for this. Thant

[aboch]

Can you pelase add some UT?

[champtar]

Hi @aboch, will do, I just got busy with other subjects

[champtar]

@aboch some other test is failing, my new test pass

        --- FAIL: TestRuleListFiltered/IPv4/returns_rules_with_specific_priority (0.00s)

=== RUN   TestLinkMacvBCQueueLen
--- PASS: TestLinkMacvBCQueueLen (0.09s)

[aboch]

LGTM