arm: handle discontig page struct between sections

If SPARSEMEM is enabled and there is a large amount of
memory, the page structures for the various sections
may not be contiguous. The code to traverse all of the
page structures in show_mem() was incorrectly assuming
all of the page structures were contiguous, causing
kernel panics in this case.

CRs-fixed: 315006
Change-Id: I5e9437c369d23f1513c73feb46623006561d15cf
Signed-off-by: Olav Haugan <ohaugan@codeaurora.org>