USB: usbmon: fix bug in mon_buff_area_shrink

This patch (as1299b) fixes a bug in an error-handling path of usbmon's
binary interface.  The storage area for URB data is divided into
fixed-size blocks.  If an URB's data can't be copied, the area
reserved for it should be decreased to the size of the truncated
information (rounded up to a block boundary).  Rounding up the amount
to be removed and subtracting it from the reserved size is definitely
the wrong thing to do.

Also, when the data for an isochronous URB can't be copied, we can
still copy the isoc packet descriptors.  In fact the current code does
copy the descriptors, but then sets the capture length to 0 so they
remain inaccessible.  The capture length should be reduced to the
length of the descriptors, not set to 0.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Acked-by: Pete Zaitcev <zaitcev@redhat.com>
CC: stable <stable@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>