net/sched: remove unneeded NULL check

There is no need to check "s".  nla_data() doesn't return NULL.  Also we
already dereferenced "s" at this point so it would have oopsed ealier if
it were NULL.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>