afs: Slight adjustments in afs_GetDCache

afs: Slight adjustments in afs_GetDCache

maxGoodSize is only used in one block, so move the decl to that block.
Adjust some of the comments to more accurately reflect what's going
on.

Reviewed-on: http://gerrit.openafs.org/6936
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Alistair Ferguson <alistair.ferguson@mac.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>
(cherry picked from commit d5c13a0f3ce7d9f0a01820ff9c12dfb1cbc12047)

Change-Id: I2ba6574b88856d2f506663eafb984a9f3bea4cf8
Reviewed-on: http://gerrit.openafs.org/7995
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>