s390x/mm: cleanup gmap_pte_op_walk()

gmap_mprotect_notify() refuses shadow gmaps. Turns out that
a) gmap_protect_range()
b) gmap_read_table()
c) gmap_pte_op_walk()

Are never called for gmap shadows. And never should be. This dates back
to gmap shadow prototypes where we allowed to call mprotect_notify() on
the gmap shadow (to get notified about the prefix pages getting removed).
This is avoided by always getting notified about any change on the gmap
shadow.

The only real function for walking page tables on shadow gmaps is
gmap_table_walk().

So, essentially, these functions should never get called and
gmap_pte_op_walk() can be cleaned up. Add some checks to callers of
gmap_pte_op_walk().

Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20171110151805.7541-1-david@redhat.com>
Reviewed-by: Janosch Frank <frankja@linux.vnet.ibm.com>
Acked-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>