MIPS: Fix ioremap() RAM check

We currently attempt to check whether a physical address range provided
to __ioremap() may be in use by the page allocator by examining the
value of PageReserved for each page in the region - lowmem pages not
marked reserved are presumed to be in use by the page allocator, and
requests to ioremap them fail.

The way we check this has been broken since commit 92923ca3 ("mm:
meminit: only set page reserved in the memblock region"), because
memblock will typically not have any knowledge of non-RAM pages and
therefore those pages will not have the PageReserved flag set. Thus when
we attempt to ioremap a region outside of RAM we incorrectly fail
believing that the region is RAM that may be in use.

In most cases ioremap() on MIPS will take a fast-path to use the
unmapped kseg1 or xkphys virtual address spaces and never hit this path,
so the only way to hit it is for a MIPS32 system to attempt to ioremap()
an address range in lowmem with flags other than _CACHE_UNCACHED.
Perhap...