MIPS: highmem: ensure that we don't use more than one page for PTEs
All PTEs used by PKMAP should be allocated in a contiguous memory area,
but we do not currently have a mechanism to enforce that, so ensure that
we don't try to allocate more entries than would fit in a single page.
Current fixed value of 1024 would not work with XPA enabled when
sizeof(pte_t)==8 and we need two pages to store pte tables.