LU-17000 coverity: Fix Dereference before null under obd_sysfs.c

LU-17000 coverity: Fix Dereference before null under obd_sysfs.c

This patch fixes Dereference before null check reported
by coverity run.

CoverityID: 404751 ("Dereference before null")

Test-Parameters: trivial
Signed-off-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Change-Id: I89bcc820244ab17a60bf1d5c86f9d6a8747b43ed
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/52903
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Timothy Day <timday@amazon.com>
Reviewed-by: Patrick Farrell <pfarrell@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>