sbp-target: fix return value check in sbp_register_configfs()

In case of error, the function target_fabric_configfs_init() returns
ERR_PTR() not NULL pointer. The NULL test in the return value check
should be replaced with IS_ERR().

dpatch engine is used to auto generated this patch.
(https://github.com/weiyj/dpatch)

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>