powerpc/32: drop get_pteptr()

Commit 8d30c14c ("powerpc/mm: Rework I$/D$ coherency (v3)") and
commit 90ac19a8 ("[POWERPC] Abolish iopa(), mm_ptov(),
io_block_mapping() from arch/powerpc") removed the use of get_pteptr()
outside of mm/pgtable_32.c

In mm/pgtable_32.c, the only user of get_pteptr() is change_page_attr()
which operates on kernel context and on lowmem pages only.

Make virt_to_kpte() available outside of mm/mem.c and use it instead
of get_pteptr(), and drop get_pteptr()

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/788378c6c3ba5c5298caab7c7f95e6c3c88244b8.1578558199.git.christophe.leroy@c-s.fr