mount.nfs: fix hang when getport() uses TCP against unavailable servers

If get_socket() can't get us an open TCP socket, we know the server is
down, so make  getport() exit early instead of hanging.  This logic is
copied from clnt_ping().

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Neil Brown <neilb@suse.de>