net: rtnetlink: fix error handling in rtnl_fill_statsinfo()

net: rtnetlink: fix error handling in rtnl_fill_statsinfo()

The clang static analyzer reports this issue
rtnetlink.c:5481:2: warning: Undefined or garbage
  value returned to caller
  return err;
  ^~~~~~~~~~

There is a function level err variable, in the
list_for_each_entry_rcu block there is a shadow
err.  Remove the shadow.

In the same block, the call to nla_nest_start_noflag()
can fail without setting an err.  Set the err
to -EMSGSIZE.

Fixes: adf8365b9317 ("net: rtnetlink: rtnl_fill_statsinfo(): Permit non-EMSGSIZE error returns")
Signed-off-by: Tom Rix <trix@redhat.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>