KVM: s390: Fix gmap_helper_zap_one_page() again

KVM: s390: Fix gmap_helper_zap_one_page() again

[ Upstream commit 2f393c228cc519ddf19b8c6c05bf15723241aa96 ]

A few checks were missing in gmap_helper_zap_one_page(), which can lead
to memory corruption in the guest under specific circumstances.

Add the missing checks.

Fixes: 5deafa27d9ae ("KVM: s390: Fix to clear PTE when discarding a swapped page")
Cc: stable@vger.kernel.org
Reported-by: Marc Hartmayer <mhartmay@linux.ibm.com>
Tested-by: Marc Hartmayer <mhartmay@linux.ibm.com>
Acked-by: Christian Borntraeger <borntraeger@linux.ibm.com>
Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
[ adapted ptep_zap_softleaf_entry() and softleaf_from_pte() calls to ptep_zap_swap_entry() and pte_to_swp_entry() ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>