waitqueue lockdep annotation V3

[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.

Changes since V2:
  - various typo fixes
  - updated comments in epoll to always refer to ep->wq.lock
  - add the new userfaultd patch from Matthew

Changes since V1:
 - remove eq->lock in epoll