usb-storage: don't access beyond the end of the sg buffer

This patch (as1038) fixes a bug in usb_stor_access_xfer_buf() and
usb_stor_set_xfer_buf() (the bug was originally found by Boaz
Harrosh): The routine must not attempt to write beyond the end of a
scatter-gather list or beyond the number of bytes requested.

This is the minimal 2.6.24 equivalent to as1035 +
as1037 (7084191d "USB:
usb-storage: don't access beyond the end of the sg buffer" +
6d512a80 "usb-storage: update earlier
scatter-gather bug fix").  Mark Glines has confirmed that it fixes
his problem.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Cc: Mark Glines <mark@glines.org>
Cc: Boaz Harrosh <bharrosh@panasas.com>
Signed-off-by: Chris Wright <chrisw@sous-sol.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>