slab: Explain why SLAB_TYPESAFE_BY_RCU reference before locking

It is not obvious to the casual user why it is absolutely necessary to
acquire a reference to a SLAB_TYPESAFE_BY_RCU structure before acquiring
a lock in that structure.  Therefore, add a comment explaining this point.

[ paulmck: Apply Vlastimil Babka feedback. ]

Signed-off-by: Paul E. McKenney <[email protected]>
Cc: Christoph Lameter <[email protected]>
Cc: Pekka Enberg <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: Joonsoo Kim <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Roman Gushchin <[email protected]>
Cc: Hyeonggon Yoo <[email protected]>
Cc: <[email protected]>
Acked-by: Vlastimil Babka <[email protected]>

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