kvm: use more precise cast and do not drop __user

Sparse complains on a call to get_compat_sigset, fix it.  The "if"
right above explains that sigmask_arg->sigset is basically a
compat_sigset_t.
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

virt/kvm/kvm_main.c

@@ -3350,7 +3350,8 @@ static long kvm_vcpu_compat_ioctl(struct file *filp,
 			if (kvm_sigmask.len != sizeof(compat_sigset_t))
 				goto out;
 			r = -EFAULT;
-			if (get_compat_sigset(&sigset, (void *)sigmask_arg->sigset))
+			if (get_compat_sigset(&sigset,
+					      (compat_sigset_t __user *)sigmask_arg->sigset))
 				goto out;
 			r = kvm_vcpu_ioctl_set_sigmask(vcpu, &sigset);
 		} else