Fix restorevol sanity check on afs_int32

Fix restorevol sanity check on afs_int32

restorevol reads various values of different lengths into an
afs_int32 and does a sanity check to ensure that there is enough
room to store the desired value length.  However, the check was
done against the wrong variable, making it ineffective.

This check is unlikely to ever trigger, but fix it just in case.

Change-Id: Ia81bd9ff9a2ce8e2d93af384c8e8c664f5434d5e
Reviewed-on: http://gerrit.openafs.org/10026
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>