Windows: refactor buf_Get() to improve readability

Windows: refactor buf_Get() to improve readability

Refactor buf_Get() by using a switch() instead of a jumble
of if() conditionals.

Improve comments to make it clear that given the current
use and implementation of cm_BufRead() from cm_dcache.c
that created buffer pages will never be populated with
actual data.

Reviewed-on: http://gerrit.openafs.org/3630
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>
Tested-by: Jeffrey Altman <jaltman@openafs.org>
(cherry picked from commit 9f584e811486da7129a61da554fae09029b0de67)

Change-Id: I32ff33df359fb7794e9cf47c2027eb48363c80eb
Reviewed-on: http://gerrit.openafs.org/3816
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>