dccp ccid-3: X truncated due to type conversion

This fixes a bug in computing the inter-packet-interval t_ipi = s/X: 

 scaled_div32(a, b) uses u32 for b, but in "scaled_div32(s, X)" the type of the
 sending rate `X' is u64. Since X is scaled by 2^6, this truncates rates greater
 than 2^26 Bps (~537 Mbps).

Using full 64-bit division now.

Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>