LU-9859 libcfs: make lnet_debugfs_symlink_def local to libcfs/modules.c

LU-9859 libcfs: 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.

Linux-commit: c4f907719736b720aa831447828809840e533371

Test-Parameters: trivial
Change-Id: I08221cfc781735451a292ba20cd35b9172fc20f2
Signed-off-by: Mr. NeilBrown <neilb@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-on: https://review.whamcloud.com/44332
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>