cifs.idmap: set a timeout on keys that it instantiates

cifs.idmap: set a timeout on keys that it instantiates

...and add a command-line option to allow the admin to tune that value.
I think this is a better way to handle this instead of trying to set the
timeouts in kernel space.

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