Poll for IO readiness events
Note: If ph is non-NULL, the client should notify when IO readiness events occur by calling fuse_notify_poll() with the specified ph.
Regardless of the number of times poll with a non-NULL ph is received, single notification is enough to clear all. Notifying more times incurs overhead but doesn't harm correctness.
The callee is responsible for destroying ph with fuse_pollhandle_destroy() when no longer in use.
Introduced in version 2.8