afs: Introduce afs_IsDCacheFresh

afs: Introduce afs_IsDCacheFresh

Numerous places in libafs check the DV of a dcache against the DV of
the vcache for the same file, in order to check if the dcache is up to
date and can be used. Consolidate all of these checks into a new
function, afs_IsDCacheFresh, to make it easier for future commits to
alter this logic.

This commit should have no visible impact; it is just code
reorganization.

Change-Id: Iedc02b0f5d7d0542ab00ff1effdde03c2a851df4
Reviewed-on: https://gerrit.openafs.org/13435
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: Andrew Deason <adeason@sinenomine.net>