atm/svc: Fix blocking in wait loop

One should not call blocking primitives inside a wait loop, since both
require task_struct::state to sleep, so the inner will destroy the
outer state.

sigd_enq() will possibly sleep for alloc_skb().  Move sigd_enq() before
prepare_to_wait() to avoid sleeping while waiting interruptibly.  You do
not actually need to call sigd_enq() after the initial prepare_to_wait()
because we test the termination condition before calling schedule().

Based on suggestions from Peter Zijlstra.

Signed-off-by: Chas Williams <chas@cmf.n4rl.navy.mil>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: David S. Miller <davem@davemloft.net>