ARM: mm: use physical addresses in highmem sanity checks
This patch modifies the highmem sanity checking code to use physical addresses
instead. This change eliminates the wrap-around problems associated with the
original virtual address based checks, and this simplifies the code a bit.
The one constraint imposed here is that low physical memory must be mapped in
a monotonically increasing fashion if there are multiple banks of memory,
i.e., x < y must => pa(x) < pa(y).