cm: address race condition in afs_QueueVCB

cm: address race condition in afs_QueueVCB

Access the vcache callback member after taking
the xvcb lock to avoid the server object from
being freed in FlushServer on another thread.

Eventually, we should have a ref count on
avc->server.

FIXES 125596

Change-Id: I9328a0b3e372353b7812d6e9f2cbea44655a3d58
Reviewed-on: http://gerrit.openafs.org/817
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>