files: remove ghost directories when a restore failed

The current code doesn't work becuase ghost files and directories
have different formats.

ghost directories are created from a root task, but they are
cleaned up from the criu process, so reg_file_info is allocated
from shared memory and is_dir is added into it.

Fixes #120

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