nfsd4: remove redundant callback probe

Both nfsd4_init_conn and alloc_init_session are probing the callback
channel, harmless but pointless.

Also, nfsd4_init_conn should probably be probing in the "unknown" case
as well.  In fact I don't see any harm to just doing it unconditionally
when we get a new backchannel connection.

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