expr: Introduce struct expr_ops::attr_policy

Similar to kernel's nla_policy, enable expressions to inform about
restrictions on attribute use. This allows the generic expression code
to perform sanity checks before dispatching to expression ops.

For now, this holds only the maximum data len which may be passed to
nftnl_expr_set().

While one may debate whether accepting e.g. uint32_t for sreg/dreg
attributes is correct, it is necessary to not break nftables.

Note that this introduces artificial restrictions on name lengths which
were caught by the kernel (if nftables didn't).

Signed-off-by: Phil Sutter <phil@nwl.cc>

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