net: remove an unneeded check

"ifa->ifa_label" is an array inside the in_ifaddr struct.  It can never
be NULL so we can remove this check.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Jiri Pirko <jiri@resnulli.us>
Signed-off-by: David S. Miller <davem@davemloft.net>