powerpc/mm: Reduce hugepd size for 8M hugepages on 8xx

Commit 55c8fc3f4930 ("powerpc/8xx: reintroduce 16K pages with HW
assistance") redefined pte_t as a struct of 4 pte_basic_t, because
in 16K pages mode there are four identical entries in the page table.
But hugepd entries for 8M pages require only one entry of size
pte_basic_t. So there is no point in creating a cache for 4 entries
page tables.

Calculate PTE_T_ORDER using the size of pte_basic_t instead of pte_t.

Define specific huge_pte helpers (set_huge_pte_at(), huge_pte_clear(),
huge_ptep_set_wrprotect()) to write the pte in a single entry instead
of using set_pte_at() which writes 4 identical entries in 16k pages
mode. Also make sure that __ptep_set_access_flags() properly handle
the huge_pte case.

Define set_pte_filter() inline otherwise GCC doesn't inline it anymore
because it is now used twice, and that gives a pretty suboptimal code
because of pte_t being a struct of 4 entries.

Those functions are also used for 512k pages which only require one
entry as well allthough replicating it four times was harmless as 512k
pages entries are spread every 128 bytes in the table.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/43050d1a0c2d6e1541cab9c1126fc80bc7015ebd.1589866984.git.christophe.leroy@csgroup.eu

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