nfsclient: fix panic in cache_enter_time()

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

This trivial patch avoids doing a cache_enter_time()
call for the current directory to avoid the panic.

Reported by: otis
Tested by: otis
Reviewed by: mjg
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D28969