kvm: Fix kvm_get_page_retry_io __gup retval check

Confusion around -EBUSY and zero (inside a BUG_ON no less).

Reported-by: Andrea Arcangeli <[e-mail]>
Signed-off-by: Andres Lagar-Cavilla <[e-mail]>
Signed-off-by: Paolo Bonzini <[e-mail]>

virt/kvm/kvm_main.c

···
 	npages = __get_user_pages(tsk, mm, addr, 1, flags, pagep, NULL,
 				  &locked);
 	if (!locked) {
-		VM_BUG_ON(npages != -EBUSY);
+		VM_BUG_ON(npages);
 
 		if (!pagep)
 			return 0;