osmo_io: Move notify_connected function to backend

This relocation is necessary as the backend (osmo_io_fd or
osmo_io_uring) requires a different approach in handling connect
notifications. As a result, a function call has been introduced to
struct iofd_backend_ops.

In a subsequent patch, the process for the osmo_io_uring backend will
be modified to handle SCTP connect notifications using poll/select.

If connect notification is requested using poll/select, the file
descriptior must be registered to osmo_fd, using osmo_fd_register. If
read / write notification is requested by application, the file
descriptior must be registered also. A flag is used prevent calling
osmo_fd_register / osmo_fd_unregister multiple times, which would cause
a crash.

Change-Id: I905ec85210570aff8addadfc9603335d04eb057a
Related: OS#5751
diff --git a/src/core/osmo_io_uring.c b/src/core/osmo_io_uring.c
index 1aa17d4..24d1e08 100644
--- a/src/core/osmo_io_uring.c
+++ b/src/core/osmo_io_uring.c
@@ -402,6 +402,7 @@
 	.write_disable = iofd_uring_write_disable,
 	.read_enable = iofd_uring_read_enable,
 	.read_disable = iofd_uring_read_disable,
+	.notify_connected = iofd_uring_write_enable,
 };
 
 #endif /* defined(__linux__) */