maple_tree: remove extra smp_wmb() from mas_dead_leaves()

The call to mte_set_dead_node() before the smp_wmb() already calls
smp_wmb() so this is not needed.  This is an optimization for the RCU mode
of the maple tree.

Link: https://lkml.kernel.org/r/20230227173632.3292573-5-surenb@google.com
Fixes: 54a611b6 ("Maple Tree: add new data structure")
Signed-off-by: Liam Howlett <Liam.Howlett@oracle.com>
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

lib/maple_tree.c

@@ -5503,7 +5503,6 @@ unsigned char mas_dead_leaves(struct ma_state *mas, void __rcu **slots,
 			break;
 
 		mte_set_node_dead(entry);
-		smp_wmb(); /* Needed for RCU */
 		node->type = type;
 		rcu_assign_pointer(slots[offset], node);
 	}