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>
FBSD: Drop afs_xvcache for vgone()

For FreeBSD, osi_TryEvictVCache was calling vgone() without dropping
afs_xvcache. Prior to aad83a30a82407bfa6ac15b49fd31d69b563e898, this
is what osi_TryEvictVCache did, and since the 'slept' pointer
represents whether we dropped xvcache (not whether we dropped glock),
it seems like this is the intention of the code.

Change-Id: Icb8cc86d972d7ca717bd91e250771d90931e1ba7
Reviewed-on: http://gerrit.openafs.org/7434
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>