s390/mm: Complete ptep_get() conversion

s390/mm: Complete ptep_get() conversion

Finalize commit c33c794828f2 ("mm: ptep_get() conversion") and
replace direct page table entry dereferencing with the proper
accessors (ptep_get(), pmdp_get(), etc.).

Override the default getter implementations even though they are
currently identical: pud_clear(), p4d_clear(), and pgd_clear()
require corresponding architecture-specific getters, but these
are not yet defined. This avoids a dependency loop.

Acked-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>