net: bpfilter: Fix type cast and pointer warnings

Fixes the following Sparse warnings:

net/bpfilter/bpfilter_kern.c:62:21: warning: cast removes address space
of expression
net/bpfilter/bpfilter_kern.c:101:49: warning: Using plain integer as
NULL pointer

Signed-off-by: Shanthosh RK <shanthosh.rk@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>