mm: Implement for_each_valid_pfn() for CONFIG_SPARSEMEM

mm: Implement for_each_valid_pfn() for CONFIG_SPARSEMEM

Introduce a pfn_first_valid() helper which takes a pointer to the PFN and
updates it to point to the first valid PFN starting from that point, and
returns true if a valid PFN was found.

This largely mirrors pfn_valid(), calling into a pfn_section_first_valid()
helper which is trivial for the !CONFIG_SPARSEMEM_VMEMMAP case, and in
the VMEMMAP case will skip to the next subsection as needed.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>