venus: fix an incorrect assert in vn_ring_write_buffer
The ring code uses 32-bit unsigned integers and relies on that their
overflow/underflow behavior is well-defined. When ring->shared.head
is
about to overflow and ring->cur
has overflowed, this expression
ring->cur + size - vn_ring_load_head(ring)
used in the assert gives an incorrect result when size
is 64-bit.