staging: lustre: mdc: avoid returning freed request

In mdc_close() if ptlrpc_request_pack() fails then set req to NULL so
that an already freed request is not returned in *request.

Signed-off-by: John L. Hammond <john.hammond@intel.com>
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-8811
Reviewed-on: https://review.whamcloud.com/23843


Reviewed-by: Patrick Farrell <paf@cray.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Signed-off-by: James Simmons <jsimmons@infradead.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>