lazy-pages: handle_exit: set uffd to negative value instead of 0

The only use for the userfault file descriptor after the process exited is
for debug logs. Using negative value instead of 0 makes logs more readable.

Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>