mm/slab: stop exporting kvfree_rcu_barrier[_on_cache]()

No module code calls these functions directly. Seems it was always the
case. Remove the exports.

Acked-by: Paul E. McKenney <paulmck@kernel.org>
Reviewed-by: Harry Yoo <harry@kernel.org>
Link: https://patch.msgid.link/20260730-unexport-barriers-v1-1-852f6641abe9@kernel.org
Signed-off-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>