tools: use fclose() after fdopen() + add newline

tools: use fclose() after fdopen() + add newline

So far we accidently used close() on the original file descriptor. (After
fdopen() the original fd is considered private and should not be used anymore.
The close operations should be performed on the new file handle. We did the
correct thing on error but not on success.) Using close() on the original fd
caused "Text file busy" errors and prevented the cloned tmpfs container from
starting.

Signed-off-by: Christian Brauner <cbrauner@suse.de>