USB: add queued-unlinks test case to usbtest driver

This patch (as1452b) adds a new test case to the usbtest driver.  Test
24 exercises the unlink-from-queue pathways in the host.  It queues a
user-specified number of bulk-OUT URBs of user-specified size, unlinks
the fourth- and second-from-last URBs in the queue, and then waits to
see if all the URBs complete in the expected way (except of course
that the unlinked URBs might complete normally, if they weren't
unlinked soon enough).

This new test has confirmed the existence of a bug in the ehci-hcd
driver, to be fixed by a separate patch.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>