Use afs_DestroyReq in afs_PrefetchNoCache()

Use afs_DestroyReq in afs_PrefetchNoCache()

Since commit 76ad941902c650a4a716168d3cbe68f62aef109f we use afs_DestroyReq()
instead of osi_Free() directly.

Also update the UKERNEL version of the function to afs_CreateReq() properly.

FIXES 134533

Reviewed-on: https://gerrit.openafs.org/13060
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
(cherry picked from commit 3ec0414f769c37a19410fbd9aefb086cb5b69e55)

Change-Id: Ic85fef48f7adb2a152b5cb4d68b8c4a43c45c06a
Reviewed-on: https://gerrit.openafs.org/13114
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>