eventfd use waitqueue lock ...

The eventfd was using the unlocked waitqueue operations, but it was
using a different lock, so poll_wait() would race with it.

This makes eventfd directly use the waitqueue lock.

Signed-off-by: Davide Libenzi <davidel@xmailserver.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>