waitqueue lockdep annotation

[Christoph Hellwig]

Hi all,

this series adds a strategic lockdep_assert_held to __wake_up_common
to ensure callers really do hold the wait_queue_head lock when calling
the unlocked wake_up variants.  It turns out epoll did not do this
for a fairly common path (hit all the time by systemd during bootup),
so the second patch fixed this instance as well.