nfscl: Add a diagnostic printf() for a "should never happen" case

When a NFSv4.1/4.2 session to the NFS server (not a pNFS DS) is
replaced, the old session should always be marked defunct by
nfsess_defunct being set non-zero.

However, the hang reported by the PR suggests that this might
be the case.

This patch adds a printf() to indicate this has somehow happened.

PR: 260011
MFC after: 2 weeks