butc: Set but unused variables

butc: Set but unused variables

Remove unused variable taskId

writeData() systematically returns 0, so make it void and adapt
call sites that assigned the return value but never used it.
Also move the function up in the file to avoid the need for a
forward declaration, and make it static since it's only used here.

Spotted by gcc 4.6

Change-Id: I2f61c0395796498175f2cb9131066b00657f99a7
Reviewed-on: http://gerrit.openafs.org/2785
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>