nfsd4: CREATE_SESSION should update backchannel immediately

nfsd4_probe_callback kicks off some work that will eventually run
nfsd4_process_cb_update and update the session flags.  In theory we
could process a following SEQUENCE call before that update happens
resulting in flags that don't accurately represent, for example, the
lack of a backchannel.

Signed-off-by: J. Bruce Fields <bfields@redhat.com>