only warn for inherited file descriptors

only warn for inherited file descriptors

As discussed in thread:

http://sourceforge.net/mailarchive/forum.php?thread_name=4E5618C3.5060507%40free.fr&forum_name=lxc-devel

We think it's better for now to only warn the user about a fd leaking into
the container. Also remove the call to readlink() as it isn't really useful
now: since the container will start anyway, the user can look into /proc/../fd
or use lsof or whatever.

Signed-off-by: Greg Kurz <gkurz@fr.ibm.com>
Signed-off-by: Daniel Lezcano <dlezcano@fr.ibm.com>