Coccinelle emits a warning about casting the return value of
kmalloc(). Coccinelle suggests removing the cast as do
kerneljanitors.

Remove cast from kmalloc() call.
Signed-off-by: NTobin C. Harding <me@tobin.cc>
Acked-by: NDon Brace <don.brace@microsemi.com>
Signed-off-by: NJens Axboe <axboe@fb.com>