s390/kvm: simplify set_guest_storage_key

Git commit ab3f285f
"KVM: s390/mm: try a cow on read only pages for key ops"
added a fixup_user_fault to set_guest_storage_key force a copy on
write if the page is mapped read-only. This is supposed to fix the
problem of differing storage keys for shared mappings, e.g. the
empty_zero_page.
But if the storage key is set before the pte is mapped the storage
key update is done on the pgste. A later fault will happily map the
shared page with the key from the pgste.

Eventually git commit 2faee8ff
"s390/mm: prevent and break zero page mappings in case of storage keys"
fixed this problem for the empty_zero_page. The commit makes sure that
guests enabled for storage keys will not use the empty_zero_page at all.

As the call to fixup_user_fault in set_guest_storage_key depends on the
order of the storage key operation vs. the fault that maps the pte
it does not really fix anything. Just remove it.
Reviewed-by: Dominik Dingel <dingel@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/mm/pgtable.c

@@ -809,30 +809,13 @@ int set_guest_storage_key(struct mm_struct *mm, unsigned long addr,
 	spinlock_t *ptl;
 	pgste_t old, new;
 	pte_t *ptep;
-	bool unlocked;
 
 	down_read(&mm->mmap_sem);
-retry:
-	unlocked = false;
 	ptep = get_locked_pte(mm, addr, &ptl);
 	if (unlikely(!ptep)) {
 		up_read(&mm->mmap_sem);
 		return -EFAULT;
 	}
-	if (!(pte_val(*ptep) & _PAGE_INVALID) &&
-	     (pte_val(*ptep) & _PAGE_PROTECT)) {
-		pte_unmap_unlock(ptep, ptl);
-		/*
-		 * We do not really care about unlocked. We will retry either
-		 * way. But this allows fixup_user_fault to enable userfaultfd.
-		 */
-		if (fixup_user_fault(current, mm, addr, FAULT_FLAG_WRITE,
-				     &unlocked)) {
-			up_read(&mm->mmap_sem);
-			return -EFAULT;
-		}
-		goto retry;
-	}
 
 	new = old = pgste_get_lock(ptep);
 	pgste_val(new) &= ~(PGSTE_GR_BIT | PGSTE_GC_BIT |