LU-9567 mgc: set cfg_instance to NULL for sptlrpc case

LU-9567 mgc: set cfg_instance to NULL for sptlrpc case

The patch for LU-9034 mapped config logs to separate mgc
devices. This change broke sptlrpc by preventing the
ablitity to pass along config log change with
lctl conf_param. The reason for this is due to the
cfg_instance field being set to an obd device pointer.
This patch sets cfg_instance back to NULL to restore
the functionality. This is only a temporary work
around so GSS can function for the 2.10 release. A
proper fix will be worked out in 2.11.

Change-Id: Ic60e6516802d3617b38df3371f52a7f27aeae8cd
Signed-off-by: James Simmons <uja.ornl@yahoo.com>
Signed-off-by: John L. Hammond <john.hammond@intel.com>
Reviewed-on: https://review.whamcloud.com/27320
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Sebastien Buisson <sbuisson@ddn.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>