mm: introduce num_pages_contiguous()

mm: introduce num_pages_contiguous()

Let's add a simple helper for determining the number of contiguous pages
that represent contiguous PFNs.

In an ideal world, this helper would be simpler or not even required.
Unfortunately, on some configs we still have to maintain (SPARSEMEM
without VMEMMAP), the memmap is allocated per memory section, and we might
run into weird corner cases of false positives when blindly testing for
contiguous pages only.

One example of such false positives would be a memory section-sized hole
that does not have a memmap. The surrounding memory sections might get
"struct pages" that are contiguous, but the PFNs are actually not.

This helper will, for example, be useful for determining contiguous PFNs
in a GUP result, to batch further operations across returned "struct
page"s. VFIO will utilize this interface to accelerate the VFIO DMA map
process.

Implementation based on Linus' suggestions to avoid new usage of
nth_page() where avoidable.

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Suggested-by: Jason Gunthorpe <jgg@ziepe.ca>
Signed-off-by: Li Zhe <lizhe.67@bytedance.com>
Co-developed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Link: https://lore.kernel.org/r/20250814064714.56485-2-lizhe.67@bytedance.com
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>