Always check return code from iod_Write

Always check return code from iod_Write

The return code from iod_Write is checked at every call site
in the file, except this one.  Check it, and return VOLSERDUMPERROR
if appropriate.

Spotted by a set but unused warning from gcc 4.6

Change-Id: I84f38a4b3b1e37c25be9c76702b0d2818058454e
Reviewed-on: http://gerrit.openafs.org/2733
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit e0708617b8f5df625b6ad62920665e704f1374f7)
Reviewed-on: http://gerrit.openafs.org/2795