FBSD: Do not vgone() in osi_VM_FlushVCache

FBSD: Do not vgone() in osi_VM_FlushVCache

osi_VM_FlushVCache just needs to remove VM references to the given
vcache; calling vgone() entirely should be unnecessary. Remove the
call to vgone() and other osi_TryEvictVCache-ish stuff, and just try
to cache_purge the vnode, like the other BSD implementations do.

Change-Id: I71d71f137c04d9ef3625f6a8ae22f0ffb90b9637
Reviewed-on: http://gerrit.openafs.org/7433
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>