NFS: advertise only supported callback netids

NFSv4.0 clients use the SETCLIENTID operation to inform NFS servers
how to contact a client's callback service.  If a server cannot
contact a client's callback service, that server will not delegate
to that client, which results in a performance loss.

Our client advertises "rdma" as the callback netid when the forward
channel is "rdma".  But our client always starts only "tcp" and
"tcp6" callback services.

Instead of advertising the forward channel netid, advertise "tcp"
or "tcp6" as the callback netid, based on the value of the
clientaddr mount option, since those are what our client currently
supports.

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=69171


Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>