nfsclient: add checks for a server returning the current directory

Commit 3fe2c68ba20f dealt with a panic in cache_enter_time() where
the vnode referred to the directory argument.
It would also be possible to get these panics if a broken
NFS server were to return the directory as an new object being
created within the directory or in a Lookup reply.

This patch adds checks to avoid the panics and logs
messages to indicate that the server is broken for the
file object creation cases.

(cherry picked from commit 3e04ab36ba5ce5cbbf6d22f17a01a391a04e465f)