fuse_lowlevel_notify_poll(): use master channel

fuse_lowlevel_notify_poll(): use master channel

In theory, a poll handle could hang around much longer than the worker
thread that creates it. Furthermore, the thread that created the
pollhandle is no more likely to call fuse_lowlevel_notify_poll() than
any other thread.

In theory, this would have kept the channel alive for much longer than
necessary. In practice, there seems to have been a bug that prevented
this - and instead allowed the channel to be destroyed while there
was still a pollhandle referring to it.

Instead of fixing this by calling fuse_chan_get() and fuse_chan_put() in
do_poll() and fuse_pollhandle_destroy(), we simply transmit poll
notifications over the master channel now.