bpfilter: fix up a sparse annotation

The __user doesn't make sense when casting to an integer type, just
switch to a uintptr_t cast which also removes the need for the __force.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Luc Van Oostenryck <luc.vanoostenryck@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>