binder: signal epoll threads of self-work

In the Linux kernel, the following vulnerability has been resolved:

binder: signal epoll threads of self-work

In (e)poll mode, threads often depend on I/O events to determine when

data is ready for consumption. Within binder, a thread may initiate a

command via BINDER_WRITE_READ without a read buffer and then make use

of epoll_wait() or similar to consume any responses afterwards.

It is then crucial that epoll threads are signaled via wakeup when they

queue their own work. Otherwise, they risk waiting indefinitely for an

event leaving their work unhandled. What is worse, subsequent commands

won't trigger a wakeup either as the thread has pending work.