afs: Create afs_SetDataVersion

afs: Create afs_SetDataVersion

Several different places in the codebase change avc->f.m.DataVersion
for a particular vcache, when we've noticed that the DV for the vcache
has changed. Consolidate all of these occurrences into a single
afs_SetDataVersion function, to make it easier to change what happens
when we notice a change in DV number.

This should incur no behavior change; it is just simple code
reorganization.

Change-Id: I5dbf2678d3c4b5a2fbef6ef045a0b5bfa8a49242
Reviewed-on: https://gerrit.openafs.org/11791
Reviewed-by: Marc Dionne <marc.c.dionne@gmail.com>
Reviewed-by: Daria Phoebe Brashear <shadow@your-file-system.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Thomas Keiser <tkeiser@gmail.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>