mm/page_alloc: reuse tail struct pages for compound devmaps

Currently memmap_init_zone_device() ends up initializing 32768 pages when
it only needs to initialize 128 given tail page reuse.  That number is
worse with 1GB compound pages, 262144 instead of 128.  Update
memmap_init_zone_device() to skip redundant initialization, detailed
below.

When a pgmap @vmemmap_shift is set, all pages are mapped at a given huge
page alignment and use compound pages to describe them as opposed to a
struct per 4K.

With @vmemmap_shift > 0 and when struct pages are stored in ram (!altmap)
most tail pages are reused.  Consequently, the amount of unique struct
pages is a lot smaller than the total amount of struct pages being mapped.

The altmap path is left alone since it does not support memory savings
based on compound pages devmap.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Joao Martins <[email protected]>
Reviewed-by: Muchun Song <[email protected]>
Cc: Christoph Hellwig <[email protected]>
Cc: Dan Williams <[email protected]>
Cc: Jane Chu <[email protected]>
Cc: Jason Gunthorpe <[email protected]>
Cc: Jonathan Corbet <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Mike Kravetz <[email protected]>
Cc: Vishal Verma <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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