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.

Reviewd by: kib
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D28987