USB: add usbfs snooping for REAP and DISCARD

This patch improves the usbfs_snoop debugging facility by adding
messages for a couple of significant events which, up to now, have not
been logged.  The events are reaping and discarding (i.e.,
cancelling) an URB.  The debugging messages include the userspace
address of the URB being reaped or discarded.

The reaping messages have to be added in four places, in order to
handle blocking and non-blocking reaps in both normal and 32-bit
compatibility mode.

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