Calculate buffer ring mask size
# DESCRIPTION
[io_uring_buf_ring_mask] calculates the appropriate size mask for a
buffer ring. *ring_entries* is the ring entries as specified in
[io_uring_register_buf_ring]**.**
# RETURN VALUE
Size mask for the buffer ring.
# SEE ALSO
[io_uring_register_buf_ring], [io_uring_buf_ring_add]