SUNRPC/cache: Allow garbage collection of invalid cache entries

If the cache entry never gets initialised, we want the garbage
collector to be able to evict it. Otherwise if the upcall daemon
fails to initialise the entry, we end up never expiring it.

Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
[ cel: resolved a merge conflict ]
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>