staging/lustre/lmv: fix potential null pointer dereference

In lmv_disconnect_mdc do procfs removal only if we actually know the name.

Reviewed-on: http://review.whamcloud.com/14605
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-6517


Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Reviewed-by: Dmitry Eremin <dmitry.eremin@intel.com>
Signed-off-by: Oleg Drokin <oleg.drokin@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>