vol: Don't assume enum is an int

The type that an enum is promoted to for arithmetic is compiler
dependent. We can't assume that it's an int, or a short.

Explicitly cast the volume state enum where clang gets upset.

Reviewed-on: http://gerrit.openafs.org/9137
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
(cherry picked from commit 304fac42b8cd4a71d0df063cafe88b2888e4d175)

Change-Id: I8c5b3493549ec88727728a7f28e3430660c673b2
Reviewed-on: http://gerrit.openafs.org/10748
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>