USB: uas: Use kzalloc instead of kmalloc

The IUs are not being fully initialised by the driver (due to the reserved
space).  Since we should be zeroing reserved fields, use kzalloc to do
it for us.

Reported-by: Luben Tuikov <ltuikov@yahoo.com>
Signed-off-by: Matthew Wilcox <willy@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>