maple_tree: memset maple_big_node as a whole

In mast_fill_bnode(), we first clear some fields of maple_big_node and set
the 'type' unconditionally before return.  This means we won't leverage
any information in maple_big_node and it is safe to clear the whole
structure.

In maple_big_node, we define slot and padding/gap in a union.  And based
on current definition of MAPLE_BIG_NODE_SLOTS/GAPS, padding is always less
than slot and part of the gap is overlapped by slot.

For example on 64bit system:

  MAPLE_BIG_NODE_SLOT is 34
  MAPLE_BIG_NODE_GAP  is 21

With this knowledge, current code may clear some space by twice. And
this could be avoid by clearing the structure as a whole.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Wei Yang <[email protected]>
Reviewed-by: Liam R. Howlett <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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