libafs: use afs_ResetVCache in flush volume data

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.

Change-Id: Ib94e6b4d28078a03d2ffd217998bbc345c4968e4
Reviewed-on: http://gerrit.openafs.org/7538
Reviewed-by: Derrick Brashear <shadow@dementix.org>
Tested-by: Derrick Brashear <shadow@dementix.org>