sunrpc: fix expiry of auth creds

Before this commit, with a large enough LRU of expired items (100), the
loop skipped all the expired items and was entirely ineffectual in
trimming the LRU list.

Fixes: 95cd6232

 ('SUNRPC: Clean up the AUTH cache code')
Signed-off-by: Dan Aloni <dan.aloni@vastdata.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>