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.

Change-Id: Ib3f5778ae32f210127537e16ecc32e1598dbefc7
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>