fix panic "dvp != vp not true" in cache_enter_time()

[rmacklem]

Summary

Juraj Lutter reported a panic "dvp != vp not true" in
cache_enter_time() called from the NFS client's
"rdirplus" function.
This is specific to an NFSv3 mount with the "rdirplus"
option. Unlike NFSv4, NFSv3 replies to ReaddirPlus
includes entries for the current directory.

This trivial patch avoids doing a cache_enter_time()
call for this case. I assume that cache_enter_time()
should not be called to make an entry for the directory
itself.

Test Plan

Reproduced the problem, then applied the patch
to see that the problem did not recur.

Also had Juraj test the patch and it fixed the
panic() for him.