arm64/hugetlb: fix CMA gigantic page order for non-4K PAGE_SIZE

For non-4K PAGE_SIZE configs, the largest gigantic huge page size is
CONT_PMD_SHIFT order. On arm64 with 64K PAGE_SIZE, the gigantic page is
16G. Therefore, one should be able to specify 'hugetlb_cma=16G' on the
kernel command line so that one gigantic page can be allocated from CMA.
However, when adding such an option the following message is produced:

hugetlb_cma: cma area should be at least 8796093022208 MiB

This is because the calculation for non-4K gigantic page order is
incorrect in the arm64 specific routine arm64_hugetlb_cma_reserve().

Fixes: abb7962a ("arm64/hugetlb: Reserve CMA areas for gigantic pages on 16K and 64K configs")
Cc: <stable@vger.kernel.org> # 5.9.x
Signed-off-by: Mike Kravetz <mike.kravetz@oracle.com>
Reviewed-by: Anshuman Khandual <anshuman.khandual@arm.com>
Link: https://lore.kernel.org/r/20211005202529.213812-1-mike.kravetz@oracle.comSigned-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/mm/hugetlbpage.c

@@ -43,7 +43,7 @@ void __init arm64_hugetlb_cma_reserve(void)
 #ifdef CONFIG_ARM64_4K_PAGES
 	order = PUD_SHIFT - PAGE_SHIFT;
 #else
-	order = CONT_PMD_SHIFT + PMD_SHIFT - PAGE_SHIFT;
+	order = CONT_PMD_SHIFT - PAGE_SHIFT;
 #endif
 	/*
 	 * HugeTLB CMA reservation is required for gigantic