arm64: hugetlb: avoid potential NULL dereference

The static analyzer in GCC 10 spotted that in huge_pte_alloc() we may
pass a NULL pmdp into pte_alloc_map() when pmd_alloc() returns NULL:

|   CC      arch/arm64/mm/pageattr.o
|   CC      arch/arm64/mm/hugetlbpage.o
|                  from arch/arm64/mm/hugetlbpage.c:10:
| arch/arm64/mm/hugetlbpage.c: In function ‘huge_pte_alloc’:
| ./arch/arm64/include/asm/pgtable-types.h:28:24: warning: dereference of NULL ‘pmdp’ [CWE-690] [-Wanalyzer-null-dereference]
| ./arch/arm64/include/asm/pgtable.h:436:26: note: in expansion of macro ‘pmd_val’
| arch/arm64/mm/hugetlbpage.c:242:10: note: in expansion of macro ‘pte_alloc_map’
|     |arch/arm64/mm/hugetlbpage.c:232:10:
|     |./arch/arm64/include/asm/pgtable-types.h:28:24:
| ./arch/arm64/include/asm/pgtable.h:436:26: note: in expansion of macro ‘pmd_val’
| arch/arm64/mm/hugetlbpage.c:242:10: note: in expansion of macro ‘pte_alloc_map’

This can only occur when the kernel cannot allocate a page, and so is
unlikely to happen in practice before other systems start failing.

We can avoid this by bailing out if pmd_alloc() fails, as we do earlier
in the function if pud_alloc() fails.

Fixes: 66b3923a ("arm64: hugetlb: add support for PTE contiguous bit")
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Reported-by: Kyrill Tkachov <kyrylo.tkachov@arm.com>
Cc: <stable@vger.kernel.org> # 4.5.x-
Cc: Will Deacon <will@kernel.org>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>