afs: minor cleanup for LINUX struct vcache

afs: minor cleanup for LINUX struct vcache

mapcnt is no longer used in either linux client and should be removed
to save a little space/sanity.

Reviewed-on: http://gerrit.openafs.org/3207
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Reviewed-by: Simon Wilkinson <sxw@inf.ed.ac.uk>
Tested-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit c120f5c9b64a1333f965125dcfc435e922b47356)
Change-Id: I33e3f312ca07e9e2f7f5453495d6695a42e2cc19
Reviewed-on: http://gerrit.openafs.org/3222
Tested-by: BuildBot <buildbot@rampaginggeek.com>