tcp_bbr: clarify that bbr_bdp() rounds up in comments

This explicitly clarifies that bbr_bdp() returns the rounded-up value of
the bandwidth-delay product and why in the comments.

Signed-off-by: Luke Hsiao <lukehsiao@google.com>
Acked-by: Soheil Hassas Yeganeh <soheil@google.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Acked-by: Priyaranjan Jha <priyarjha@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>