Optimize arc_l2c_only lists assertions

It is very expensive and not informative to call multilist_is_empty()
for each arc_change_state() on debug builds to check for impossible.
Instead implement special index function for arc_l2c_only->arcs_list,
multilists, panicking on any attempt to use it.

Reviewed-by: Mark Maybee <[email protected]>
Reviewed-by: Brian Behlendorf <[email protected]>
Signed-off-by: Alexander Motin <[email protected]>
Closes #12421

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