Merge branch 'tc-bind_class-hook'

Zhengchao Shao says:

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refactor duplicate codes in bind_class hook function

All the bind_class callback duplicate the same logic, so we can refactor
them. First, ensure n arg not empty before call bind_class hook function.
Then, add tc_cls_bind_class() helper. Last, use tc_cls_bind_class() in
filter.
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Signed-off-by: David S. Miller <davem@davemloft.net>