FBSD: do not FlushAllVCaches

FBSD: do not FlushAllVCaches

In normal operation, any AFS vcache with associated data will have
an associated vnode, which will be on the list of vnodes associated
with the /afs mountpoint.  We already call FreeBSD's vflush() in
our afs_unmount, which walks the list of vnodes associated with the
mountpoint and calls vgonel() on them, which calls VOP_CLOSE and
VOP_RECLAIM on the vnode.  Our implementation of VOP_RECLAIM already
calls FlushVCache, so in normal operation, FlushAllVCaches() will
be a no-op.
However, in the presence of bugs, it is actively harmful, causing
panics.  For example, if a vnode has been reclaimed but FlushVCache
failed (which we cannot report back since the VFS will panic in this
case), and we attempt to flush it again, the associated vnode has
already been cleaned up and we will panic.  Likewise if our list of
vcaches becomes corrupt and has a vcache with bad or missing vnode
for some other reason, we will panic.

Since there is no gain in normal operation and abnormal operation
is more likely to panic than save data, skip the extra flush.

Change-Id: Id227ca74f4036c1c1f40a41a922e73198f16f958
Reviewed-on: http://gerrit.openafs.org/4847
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>