afs: Negate codes using a clear, standard method

afs: Negate codes using a clear, standard method

The bulk of our code uses 'code = -code' to negate an error code.
Use this, rather than 'code *= -1', as the latter form makes my
head hurt.

Change-Id: I578fbd7c123c37d89ceb1a6373409feb8b619d86
Reviewed-on: http://gerrit.openafs.org/6511
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>