add safetybelt checks for cache_enter_time() when an NFS server sends a bogus reply

[rmacklem]

Summary

D28969 dealt with a panic in cache_enter_time() where
the vnode referred to the directory.
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.

This patch adds checks to avoid the panics and logs
messages to indicate that the server is broken.

Test Plan

Ran connectathon test suite for various mount options.