eventfd: Export eventfd_ctx_do_read()

[ Upstream commit 28f1326710555bbe666f64452d08f2d7dd657cae ]

Where events are consumed in the kernel, for example by KVM's
irqfd_wakeup() and VFIO's virqfd_wakeup(), they currently lack a
mechanism to drain the eventfd's counter.

Since the wait queue is already locked while the wakeup functions are
invoked, all they really need to do is call eventfd_ctx_do_read().

Add a check for the lock, and export it for them.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Message-Id: <20201027135523.646811-2-dwmw2@infradead.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Stable-dep-of: 758b49204781 ("eventfd: prevent underflow for eventfd semaphores")
Signed-off-by: Sasha Levin <sashal@kernel.org>