powerpc/mm: Fix build break with STRICT_MM_TYPECHECKS && DEBUG_PAGEALLOC

If both STRICT_MM_TYPECHECKS and DEBUG_PAGEALLOC are enabled, the code
in kernel_map_linear_page() is built, and so we fail with:

  arch/powerpc/mm/hash_utils_64.c:1478:2:
  error: incompatible type for argument 1 of 'htab_convert_pte_flags'

Fix it by using pgprot_val().

Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>