OpenBSD: Fix use of mstat Length field in osi_vm.c

OpenBSD: Fix use of mstat Length field in osi_vm.c

The code for osi_VM_FlushPages() in osi_vm.c for OpenBSD incorrectly
tries to access field "m.Length" in the vcache structure "avc"
instead of "f.m.Length".

Reviewed-on: http://gerrit.openafs.org/3417
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit 35daebc577ab11a5fd80d678764a3a681cc5f2f3)
Change-Id: Iffbccfdf0f066d0cb02bb39146afd9520c9bc277
Reviewed-on: http://gerrit.openafs.org/3454