net: SOCKWQ_ASYNC_WAITDATA optimizations

SOCKWQ_ASYNC_WAITDATA is set/cleared in sk_wait_data()
and equivalent functions, so that sock_wake_async() can send
a SIGIO only when necessary.

Since these atomic operations are really not needed unless
socket expressed interest in FASYNC, we can omit them in most
cases.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>