Initialise a buffer ring
# DESCRIPTION
[io_uring_buf_ring_init] initialises *br* so that it is ready to be
used. It may be called after [io_uring_register_buf_ring] but must
be called before the buffer ring is used in any other way.
# RETURN VALUE
None
# NOTES
Unless manual setup is needed, it's recommended to use
[io_uring_setup_buf_ring] as it provides a simpler way to setup a
provided buffer ring.
# SEE ALSO
[io_uring_register_buf_ring], [io_uring_setup_buf_ring],
[io_uring_buf_ring_add] [io_uring_buf_ring_advance],
[io_uring_buf_ring_cq_advance]