io_uring: register ring from loop thread
io_uring: register ring from loop thread

Summary: it is important to register the fd from the event base thread to allow moving to a different thread (once).

Reviewed By: yfeldblum

Differential Revision: D37921702

fbshipit-source-id: 455ef8c2a4ddbf2c68fbcdb10b9b41914e8c3140

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly