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

(cherry picked from commit a7bb120f8b8735bc4c417b3c8fc2308c3d2964aa)