x86: fix frame table initialization when hotplug memory regions were detected

x86: fix frame table initialization when hotplug memory regions were detected

max_idx is not a pdx, and hence needs to be converted to one in all
cases where it is being passed to pdx_to_page().

Also, just like for max_pdx, the conversion result of max_idx may
point into an address space hole, and hence it must not be used
directly as an argument to pdx_to_page(). Note that this doesn't apply
to the arguments passed to memset(), as the size argument would be
zero in the case of hitting an address space hole.

Signed-off-by: Jan Beulich <jbeulich@novell.com>