maple_tree: introduce mas_wr_prealloc_setup()

Introduce a helper function, mas_wr_prealoc_setup(), that will set up a
maple write state in order to start a walk of a maple tree.

Link: https://lkml.kernel.org/r/20240814161944.55347-3-sidhartha.kumar@oracle.com


Signed-off-by: Sidhartha Kumar <sidhartha.kumar@oracle.com>
Cc: Liam R. Howlett <Liam.Howlett@oracle.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>