lxc-checkconfig: Only check probed modules if /proc/modules exists

lxc-checkconfig: Only check probed modules if /proc/modules exists

Kernels can be built with CONFIG_MODULES=n which results in
is_probed() telling the module isn't probed and lsmod spamming the
following to stderr each time it's called:

  libkmod: kmod_module_new_from_loaded: could not open /proc/modules: No such file or directory
  Error: could not get list of modules: No such file or directory

Fix this by "stubbing" is_probed() when /proc/modules doesn't exist
as it's always called after a is_enabled() anyway.

Signed-off-by: Jami Kettunen <jami.kettunen@protonmail.com>