FBSD: Add osi_fbsd_checkinuse

FBSD: Add osi_fbsd_checkinuse

Add the osi_fbsd_checkinuse function, which contains code common to
the FreeBSD osi_TryEvictVCache and osi_VM_FlushVCache. Implement the
latter two in terms of osi_fbsd_checkinuse.

This commit should incur no behavior changes. This is just a
reorganization so future commits can change the implementations of
osi_TryEvictVCache and osi_VM_FlushVCache.

Change-Id: I42df9d6efb7b573bd933d0bf04924f668a3608da
Reviewed-on: http://gerrit.openafs.org/7432
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Derrick Brashear <shadow@dementix.org>