s390/pgtable: fix mprotect for single-threaded KVM guests

For a single-threaded KVM guest ptep_modify_prot_start will not use
IPTE, the invalid bit will therefore not be set. If DEBUG_VM is set
pgste_set_key called by ptep_modify_prot_commit will complain about
the missing invalid bit. ptep_modify_prot_start should set the
invalid bit in all cases.

Cc: stable@vger.kernel.org # 3.10+
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>