alpha: align hugetlb mappings in arch_get_unmapped_area()

hugetlb_get_unmapped_area() does not align the address itself; it
delegates to mm_get_unmapped_area_vmaflags(), and the huge page
alignment is applied by generic_get_unmapped_area() and
generic_get_unmapped_area_topdown() via info.align_mask.

Alpha defines HAVE_ARCH_UNMAPPED_AREA and supplies its own
arch_get_unmapped_area(), which never sets align_mask. An
mmap(MAP_HUGETLB) with no address hint would therefore return a merely
page-aligned address.

That has been the architecture's job since commit 7bd3f1e1a9ae ("mm:
make hugetlb mappings go through mm_get_unmapped_area_vmflags"), whose
series converted the generic code along with x86, s390, sparc and
powerpc. LoongArch, which also has its own arch_get_unmapped_area(), was
missed and hit a BUG in LTP's hugefork02 until commit 3109d5ff484b
("LoongArch: Set hugetlb mmap base address aligned with pmd size").

Pass the file down to arch_get_unmapped_area_1() and set align_mask for
hugetlbfs mappings, as x86, s390, sparc and loongarch already do. This
is a no-op until alpha gains hugetlb support.

Reviewed-by: Magnus Lindholm <linmag7@gmail.com>
Tested-by: Magnus Lindholm <linmag7@gmail.com>
Signed-off-by: Matt Turner <mattst88@gmail.com>
Link: https://lore.kernel.org/r/20261007-alpha-hugepages-v2-5-8e108af2d961@gmail.com
Signed-off-by: Magnus Lindholm <linmag7@gmail.com>

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