NFSv4: Fix the return value for nfs_callback_start_svc

returning PTR_ERR(cb_info->task) just after we have set it to
NULL looks like a typo...

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
Cc: Stanislav Kinsbursky <skinsbursky@parallels.com>