libafs: use afs_ResetVCache in flush volume data

Remove some code duplication by using afs_ResetVCache
in the flush volume data pioctl. Adds a flag to
ResetVCache to avoid unneeded calls to purge dnlc
when reseting all the vcaches in a volume.

Adds freeing of vcache link data in the flush volume
data pioctl.

Reviewed-on: http://gerrit.openafs.org/7538
Reviewed-by: Derrick Brashear <shadow@dementix.org>
Tested-by: Derrick Brashear <shadow@dementix.org>
(cherry picked from commit 36c6c46bd081e0278611c3bb6ae25dd765f10648)

Change-Id: I0c4a001c20633a404c299359914f2368b4492901
Reviewed-on: http://gerrit.openafs.org/9388
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>
Tested-by: BuildBot <buildbot@rampaginggeek.com>