USB storage: sg chaining support

[PATCH] USB storage: sg chaining support

Modify usb_stor_access_xfer_buf() to take a pointer to an sg
entry pointer, so we can keep track of that instead of passing
around an integer index (which we can't use when dealing with
multiple scatterlist arrays).

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>