osmo_io: Use poll/select to notify socket connection at osmo_io_uring.c

In order to receive a connect notification from SCTP socket,
poll/select event must be used instead of a write notification via
io_uring completion event.

Once the connect notification has been received, subsequent write
notifications via io_uring are used.

Change-Id: I4eca9ea72beb0d6ea4d44cce81ed620033f07270
Related: OS#5751
diff --git a/src/core/osmo_io_internal.h b/src/core/osmo_io_internal.h
index af47a3d..a4b0749 100644
--- a/src/core/osmo_io_internal.h
+++ b/src/core/osmo_io_internal.h
@@ -104,6 +104,8 @@
 			void *read_msghdr;
 			void *write_msghdr;
 			/* TODO: index into array of registered fd's? */
+			/* osmo_fd for non-blocking connect handling */
+			struct osmo_fd connect_ofd;
 		} uring;
 	} u;
 };