KVM: s390: Fix to clear PTE when discarding a swapped page

KVM: s390: Fix to clear PTE when discarding a swapped page

KVM run fails when guests with 'cmm' cpu feature and host are
under memory pressure and use swap heavily. This is because
npages becomes ENOMEN (out of memory) in hva_to_pfn_slow()
which inturn propagates as EFAULT to qemu. Clearing the page
table entry when discarding an address that maps to a swap
entry resolves the issue.

Fixes: 200197908dc4 ("KVM: s390: Refactor and split some gmap helpers")
Cc: stable@vger.kernel.org
Suggested-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Signed-off-by: Gautam Gala <ggala@linux.ibm.com>
Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>