decnet: fix sparse warnings: symbol shadows an earlier one

Impact: Remove redundant variable declarations, resp. rename
inner scope variable.

Fix this sparse warnings:
  net/decnet/af_decnet.c:1252:40: warning: symbol 'skb' shadows an earlier one
  net/decnet/af_decnet.c:1223:24: originally declared here
  net/decnet/af_decnet.c:1582:29: warning: symbol 'val' shadows an earlier one
  net/decnet/af_decnet.c:1527:22: originally declared here
  net/decnet/dn_dev.c:687:21: warning: symbol 'err' shadows an earlier one
  net/decnet/dn_dev.c:670:13: originally declared here
  net/decnet/sysctl_net_decnet.c:182:21: warning: symbol 'len' shadows an earlier one
  net/decnet/sysctl_net_decnet.c:173:16: originally declared here

Signed-off-by: Hannes Eder <hannes@hanneseder.net>
Signed-off-by: David S. Miller <davem@davemloft.net>