[NET]: sparse warning fixes

Fix a bunch of sparse warnings. Mostly about 0 used as
NULL pointer, and shadowed variable declarations.
One notable case was that hash size should have been unsigned.

Signed-off-by: Stephen Hemminger <shemminger@linux-foundation.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/ndisc.c

@@ -256,7 +256,7 @@ static struct ndisc_options *ndisc_parse_options(u8 *opt, int opt_len,
 			break;
 		case ND_OPT_PREFIX_INFO:
 			ndopts->nd_opts_pi_end = nd_opt;
-			if (ndopts->nd_opt_array[nd_opt->nd_opt_type] == 0)
+			if (!ndopts->nd_opt_array[nd_opt->nd_opt_type])
 				ndopts->nd_opt_array[nd_opt->nd_opt_type] = nd_opt;
 			break;
 #ifdef CONFIG_IPV6_ROUTE_INFO