powerpc/32: refactor pmd_offset(pud_offset(pgd_offset...

At several places pmd pointer is retrieved through the same action:

	pmd = pmd_offset(pud_offset(pgd_offset(mm, addr), addr), addr);

or

	pmd = pmd_offset(pud_offset(pgd_offset_k(addr), addr), addr);

Refactor this by implementing two helpers pmd_ptr() and pmd_ptr_k()

This will help when adding the p4d level.

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/7b065c5be35726af4066cab238ee35cabceda1fa.1578558199.git.christophe.leroy@c-s.fr