hugetlb: pte_alloc_huge() to replace huge pte_alloc_map()

Some architectures can have their hugetlb pages down at the lowest PTE
level: their huge_pte_alloc() using pte_alloc_map(), but without any
following pte_unmap().  Since none of these arches uses CONFIG_HIGHPTE,
this is not seen as a problem at present; but would become a problem if
forthcoming changes were to add an rcu_read_lock() into pte_offset_map(),
with the rcu_read_unlock() expected in pte_unmap().

Similarly in their huge_pte_offset(): pte_offset_kernel() is good enough
for that, but it's probably less confusing if we define pte_offset_huge()
along with pte_alloc_huge().  Only define them without CONFIG_HIGHPTE: so
there would be a build error to signal if ever more work is needed.

For ease of development, define these now for 6.4-rc1, ahead of any use:
then architectures can integrate patches using them, independent from mm.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Hugh Dickins <[email protected]>
Reviewed-by: Mike Kravetz <[email protected]>
Cc: Muchun Song <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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