powerpc/booke: Only check for hugetlb in flush if vma != NULL

And in flush_hugetlb_page(), don't check whether vma is NULL after
we've already dereferenced it.

This was found by Dan using static analysis as described here:
https://lists.ozlabs.org/pipermail/linuxppc-dev/2013-November/113161.html

We currently get away with this because the callers that currently pass
NULL for vma seem to be 32-bit-only (e.g. highmem, and
CONFIG_DEBUG_PGALLOC in pgtable_32.c)  Hugetlb is currently 64-bit only,
so we never saw a NULL vma here.

Signed-off-by: Scott Wood <scottwood@freescale.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>

arch/powerpc/mm/tlb_nohash.c

@@ -305,7 +305,7 @@ void __flush_tlb_page(struct mm_struct *mm, unsigned long vmaddr,
 void flush_tlb_page(struct vm_area_struct *vma, unsigned long vmaddr)
 {
 #ifdef CONFIG_HUGETLB_PAGE
-	if (is_vm_hugetlb_page(vma))
+	if (vma && is_vm_hugetlb_page(vma))
 		flush_hugetlb_page(vma, vmaddr);
 #endif