cifs: add new field to track the last access time of cfid

The handlecache code today tracks the time at which dir lease was
acquired and the laundromat thread uses that to check for old
entries to cleanup.

However, if a directory is actively accessed, it should not
be chosen to expire first.

This change adds a new last_access_time field to cfid and
uses that to decide expiry of the cfid.

Signed-off-by: Shyam Prasad N <sprasad@microsoft.com>
Signed-off-by: Steve French <stfrench@microsoft.com>