maple_tree: use maple copy node for mas_wr_split()

maple_tree: use maple copy node for mas_wr_split()

Instead of using the maple big node, use the maple copy node for reduced
stack usage and aligning with mas_wr_rebalance() and
mas_wr_spanning_store().

Splitting a node is similar to rebalancing, but a new evaluation of when
to ascend is needed.  The only other difference is that the data is pushed
and never rebalanced at each level.

The testing must also align with the changes to this commit to ensure the
test suite continues to pass.

Link: https://lkml.kernel.org/r/20260130205935.2559335-27-Liam.Howlett@oracle.com
Signed-off-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Cc: Alice Ryhl <aliceryhl@google.com>
Cc: Andrew Ballance <andrewjballance@gmail.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Christian Kujau <lists@nerdbynature.de>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: SeongJae Park <sj@kernel.org>
Cc: Sidhartha Kumar <sidhartha.kumar@oracle.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>