If the user's login time is newer than the cache's timestamp,
we expect the cache may be stale and need to clear.
The stale cache will remain in the list's tail if no other
users operate on that inode.
Once the user accesses the inode, the stale cache will be
returned in rcu path.
Signed-off-by: NChengen Du <chengen.du@canonical.com>
Signed-off-by: NTrond Myklebust <trond.myklebust@hammerspace.com>