maple_tree: refine ma_state init from mas_start()

maple_tree: refine ma_state init from mas_start()

If mas->node is an MAS_START, there are three cases, and they all assign
different values to mas->node and mas->offset.  So there is no need to set
them to a default value before updating.

Update them directly to make them easier to understand and for better
readability.

Link: https://lkml.kernel.org/r/20221221060058.609003-7-vernon2gm@gmail.com
Cc: <Stable@vger.kernel.org>
Fixes: 54a611b60590 ("Maple Tree: add new data structure")
Signed-off-by: Vernon Yang <vernon2gm@gmail.com>
Signed-off-by: Liam R. Howlett <Liam.Howlett@oracle.com>