cifs: set a timeout on keys in cifs.idmap cache

cifs: set a timeout on keys in cifs.idmap cache

We want to allow unused idmap keys to eventually be purged from the
cache. Implement that by simply setting a timeout on the key whenever we
use it. We use the same default timeout as the NFS idmap cache, and add
a module parm to make it tunable.

Reviewed-by: Shirish Pargaonkar <shirishpargaonkar@gmail.com>
Signed-off-by: Jeff Layton <jlayton@redhat.com>