vol: Don't assume enum is an int

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.

Change-Id: Iab285d3a04dac698797ab0df2337aee75e3876b2
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>