powerpc/mm: Fix hugetlb page initialization

This patch fixes a regression by using correct kernel config variable
for HUGETLB_PAGE_SIZE_VARIABLE.

Without this huge pages are disabled during kernel boot.
[0.309496] hugetlbfs: disabling because there are no supported hugepage sizes

Fixes: c5710cd ("powerpc/mm: cleanup HPAGE_SHIFT setup")
Reported-by: Sachin Sant <sachinp@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Tested-by: Sachin Sant <sachinp@linux.ibm.com>
Reviewed-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/mm/hugetlbpage.c

@@ -640,7 +640,7 @@ static int __init hugetlbpage_init(void)
 			pgtable_cache_add(PTE_T_ORDER);
 	}
 
-	if (IS_ENABLED(HUGETLB_PAGE_SIZE_VARIABLE))
+	if (IS_ENABLED(CONFIG_HUGETLB_PAGE_SIZE_VARIABLE))
 		hugetlbpage_init_default();
 
 	return 0;