net: Generalise wq_has_sleeper helper

The memory barrier in the helper wq_has_sleeper is needed by just
about every user of waitqueue_active.  This patch generalises it
by making it take a wait_queue_head_t directly.  The existing
helper is renamed to skwq_has_sleeper.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>