staging/lustre: release MGC device if connect fails

In lustre_fill_super() if lustre_start_mgc() fails then call
lustre_common_put_super() to release a reference on the MGC device
attached to the LSI.

Signed-off-by: John L. Hammond <john.hammond@intel.com>
Reviewed-on: http://review.whamcloud.com/20851
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-8297


Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Mike Pershin <mike.pershin@intel.com>
Signed-off-by: Oleg Drokin <green@linuxhacker.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>