tcmu: fix error return code in tcmu_configure_device()

Fix to return error code -ENOMEM from the kzalloc() error handling
case instead of 0, as done elsewhere in this function.

Fixes: 80eb8761

 ("tcmu: allow max block and global max blocks to be settable")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Acked-by: Mike Christie <mchristi@redhat.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>