staging: lustre: make lnet_debugfs_symlink_def local to libcfs/modules.c

This type is only used in libcfs/module.c, so make it local to there.
If any other module ever wanted to add its own symlinks,
it would probably be easiest to export lnet_debugfs_root
and just call debugfs_create_symlink as required.

Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>