uffd: handle errors from userfaultfd()

>>> >>>     CID 161312:  Error handling issues  (NEGATIVE_RETURNS)
>>> >>>     "task_args->uffd" is passed to a parameter that cannot be negative.

Signed-off-by: Andrew Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>