maple_tree: Clean up mas_wr_node_store()

maple_tree: Clean up mas_wr_node_store()

The new_end does not need to be passed in as the data is already being
checked.  This allows for other areas to skip getting the node new_end
in the calling function.

The type was incorrectly void * instead of void __rcu *, which isn't an
issue but is technically incorrect.

Move the variable assignment to after the declarations to clean up the
initial setup.

Ensure there is something to copy before calling memcpy().

Signed-off-by: Liam R. Howlett <Liam.Howlett@oracle.com>