maple_tree: introduce maple_copy node and use it in mas_spanning_rebalance()

Introduce an internal-memory only node type called maple_copy to
facilitate internal copy operations.  Use it in mas_spanning_rebalance()
for just the leaf nodes.  Initially, the maple_copy node is used to
configure the source nodes and copy the data into the big_node.

The maple_copy contains a list of source entries with start and end
offsets.  One of the maple_copy entries can be itself with an offset of 0
to 2, representing the data where the store partially overwrites entries,
or fully overwrites the entry.  The side effect is that the source nodes
no longer have to worry about partially copying the existing offset if it
is not fully overwritten.

This is in preparation of removal of the maple big_node, but for the time
being the data is copied to the big node to limit the change size.

Link: https://lkml.kernel.org/r/20260130205935.2559335-12-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>

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