Windows: Remove Driver Verifier warning of memory leak

Work items are now queued in an AFSRedir.sys data structure
and are not freed when AFSRedirLib.sys is unloaded.  Therefore,
do allocate the memory with AFSLibExAllocatePoolWithTag().
The allocation by AFSLibExAllocatePoolWithTag results in a false
Driver Verifier warning of a memory leak on unload.

Change-Id: I268c4b6d20090e88114456a24b7648eef59f6289
Reviewed-on: http://gerrit.openafs.org/9641
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Peter Scott <pscott@kerneldrivers.com>
Reviewed-by: Rod Widdowson <rdw@steadingsoftware.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Tested-by: Jeffrey Altman <jaltman@your-file-system.com>