dmapool: consolidate page initialization

Various fields of the dma pool are set in different places. Move it all
to one function.

Link: https://lkml.kernel.org/r/20230126215125.4069751-9-kbusch@meta.com


Signed-off-by: Keith Busch <kbusch@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Tony Battersby <tonyb@cybernetics.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>