powerpc/booke: Fix boot crash due to null hugepd

On 32-bit book-e machines, hugepd_ok() no longer takes into account null
hugepd values, causing this crash at boot:

  Unable to handle kernel paging request for data at address 0x80000000
  ...
  NIP [c0018378] follow_huge_addr+0x38/0xf0
  LR [c001836c] follow_huge_addr+0x2c/0xf0
  Call Trace:
   follow_huge_addr+0x2c/0xf0 (unreliable)
   follow_page_mask+0x40/0x3e0
   __get_user_pages+0xc8/0x450
   get_user_pages_remote+0x8c/0x250
   copy_strings+0x110/0x390
   copy_strings_kernel+0x2c/0x50
   do_execveat_common+0x478/0x630
   do_execve+0x2c/0x40
   try_to_run_init_process+0x18/0x60
   kernel_init+0xbc/0x110
   ret_from_kernel_thread+0x5c/0x64

This impacts all nxp (ex-freescale) 32-bit booke platforms.

This was caused by the change of hugepd_t.pd from signed to unsigned,
and the update to the nohash version of hugepd_ok(). Previously
hugepd_ok() could exclude all non-huge and NULL pgds using > 0, whereas
now we need to explicitly check that the value is not zero and also that
PD_HUGE is *clear*.

This isn't protected by the pgd_none() check in __find_linux_pte_or_hugepte()
because on 32-bit we use pgtable-nopud.h, which causes the pgd_none()
check to be always false.

Fixes: 20717e1f ("powerpc/mm: Fix little-endian 4K hugetlb")
Cc: stable@vger.kernel.org # v4.7+
Reported-by: Madalin-Cristian Bucur <madalin.bucur@nxp.com>
Signed-off-by: Laurentiu Tudor <laurentiu.tudor@nxp.com>
[mpe: Flesh out change log details.]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/include/asm/nohash/pgtable.h

@@ -230,7 +230,7 @@ static inline int hugepd_ok(hugepd_t hpd)
 	return ((hpd_val(hpd) & 0x4) != 0);
 #else
 	/* We clear the top bit to indicate hugepd */
-	return ((hpd_val(hpd) & PD_HUGE) ==  0);
+	return (hpd_val(hpd) && (hpd_val(hpd) & PD_HUGE) == 0);
 #endif
 }