cifs: add a cache= option to better describe the different cache flavors

Currently, we have several mount options that control cifs' cache
behavior, but those options aren't considered to be mutually exclusive.
The result is poorly-defined when someone specifies more than one of
these options at mount time.

Fix this by adding a new cache= mount option that will supercede
"strictcache", and "forcedirectio". That will help make it clear that
these options are mutually exclusive. Also, change the legacy options to
be mutually exclusive too, to ensure that users don't get surprises.

Reviewed-by: Pavel Shilovsky <piastry@etersoft.ru>
Signed-off-by: Jeff Layton <jlayton@redhat.com>