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.

Reviewed-on: http://gerrit.openafs.org/2725
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit b2d067a913d3c2e5dbfe756b7752ade308f967c1)

Change-Id: I2e2d7ca4986af417c3298c3cfff8f087d795bc53
Reviewed-on: http://gerrit.openafs.org/5750
Reviewed-by: Derrick Brashear <shadow@dementix.org>
Tested-by: Derrick Brashear <shadow@dementix.org>