ARM: 6913/1: sparsemem: allow pfn_valid to be overridden when using SPARSEMEM

In commit eb33575c ("[ARM] Double check memmap is actually valid with a
memmap has unexpected holes V2"), a new function, memmap_valid_within,
was introduced to mmzone.h so that holes in the memmap which pass
pfn_valid in SPARSEMEM configurations can be detected and avoided.

The fix to this problem checks that the pfn <-> page linkages are
correct by calculating the page for the pfn and then checking that
page_to_pfn on that page returns the original pfn. Unfortunately, in
SPARSEMEM configurations, this results in reading from the page flags to
determine the correct section. Since the memmap here has been freed,
junk is read from memory and the check is no longer robust.

In the best case, reading from /proc/pagetypeinfo will give you the
wrong answer. In the worst case, you get SEGVs, Kernel OOPses and hung
CPUs. Furthermore, ioremap implementations that use pfn_valid to
disallow the remapping of normal memory will break.

This patch allows architectures to provide their own pfn_valid function
instead of using the default implementation used by sparsemem. The
architecture-specific version is aware of the memmap state and will
return false when passed a pfn for a freed page within a valid section.

Acked-by: Mel Gorman <mgorman@suse.de>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Tested-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>