DAFS: raise vhashsize limit

DAFS: raise vhashsize limit

Raise the maximum specifiable vhashsize to 28 (from 14). Specifying a
vhashsize over 14 can be reasonable if you expect to have a few
million volumes on a fileserver.

Change-Id: I90b17209fa25032f6e2bdec9abf80e578fc9b486
Reviewed-on: http://gerrit.openafs.org/2725
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>