nfscl: Add a console message for session recovery

The NFSv4.1/4.2 client does recovery when it receives a
NFSERR_BADSESSION reply from the server. If the server has
not rebooted, this is often caused by multiple clients using
the same /etc/hostid and, as such, not being recognized as
different clients by the server.

This trivial patch adds a console message to suggest that
client's /etc/hostid's need to be checked for uniqueness.

MFC after: 2 weeks