mm/mm_init: Use for_each_valid_pfn() in init_unavailable_range()

mm/mm_init: Use for_each_valid_pfn() in init_unavailable_range()

Currently, memmap_init initializes pfn_hole with 0 instead of
ARCH_PFN_OFFSET. Then init_unavailable_range will start iterating each
page from the page at address zero to the first available page, but it
won't do anything for pages below ARCH_PFN_OFFSET because pfn_valid
won't pass.

If ARCH_PFN_OFFSET is very large (e.g., something like 2^64-2GiB if the
kernel is used as a library and loaded at a very high address), the
pointless iteration for pages below ARCH_PFN_OFFSET will take a very
long time, and the kernel will look stuck at boot time.

Use for_each_valid_pfn() to skip the pointless iterations.

Reported-by: Ruihan Li <lrh2000@pku.edu.cn>
Suggested-by: Mike Rapoport <rppt@kernel.org>
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>