Prevent VLRUQ race in ShakeLooseVCaches

Prevent VLRUQ race in ShakeLooseVCaches

When ShakeLooseVCaches is called from afs_Daemon, the xvcache lock
is not held. This means that if the GLOCK is dropped for any reason
(for example, whilst purging the dentry cache), then
ShakeLooseVCaches can be raced, end we can end up attempting to
flush the same vcache twice.

The symptoms of this in Linux are that we oops in clear_inode.

Get the xvcache lock in afs_Daemon(), before calling
ShakeLooseVCaches. Also, remove the conditional GLOCK code from
that function. If we don't have the GLOCK on entry, then we're really
in trouble (and both code paths - afs_Daemon and afs_NewVCache should
get the GLOCK for us, anyway)

FIXES 125589

Change-Id: I3fe5b41a661cd162ec73c51492925ad87c6d4c13
Reviewed-on: http://gerrit.openafs.org/781
Reviewed-by: Marc Dionne <marc.c.dionne@gmail.com>
Tested-by: Derrick Brashear <shadow@dementia.org>
Reviewed-by: Derrick Brashear <shadow@dementia.org>