sfq: deadlock in error path

The change to allow divisor to be a parameter (in 2.6.38-rc1)
 commit 817fb15d
introduced a possible deadlock caught by sparse.

The scheduler tree lock was left locked in the case of an incorrect
divisor value. Simplest fix is to move test outside of lock
which also solves problem of partial update.

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Acked-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>