maple_tree: add cp_is_new_root() helper

maple_tree: add cp_is_new_root() helper

Add a helper to do what is needed when the maple copy node contains a new
root node.  This is useful for future commits and is self-documenting
code.

[Liam.Howlett@oracle.com: remove warnings on older compilers]
Link: https://lkml.kernel.org/r/malwmirqnpuxqkqrobcmzfkmmxipoyzwfs2nwc5fbpxlt2r2ej@wchmjtaljvw3
[akpm@linux-foundation.org: s/cp->slot[0]/&cp->slot[0]/, per Liam]
Link: https://lkml.kernel.org/r/20260130205935.2559335-22-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>