[DCCP] ccid3: Fix bug in calculation of send rate

[DCCP] ccid3: Fix bug in calculation of send rate

The main object of this patch is the following bug:
 ==> In ccid3_hc_tx_packet_recv, the parameters p and X_recv were updated
     _after_ the send rate was calculated. This is clearly an error and is
     resolved by re-ordering statements.

In addition,
  * r_sample is converted from u32 to long to check whether the time difference
    was negative (it would otherwise be converted to a large u32 value)
  * protection against RTT=0 (this is possible) is provided in a further patch
  * t_elapsed is also converted to long, to match the type of r_sample
  * adds a a more debugging information regarding current send rates
  * various trivial comment/documentation updates

Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
Acked-by: Ian McDonald <ian.mcdonald@jandi.co.nz>
Signed-off-by: Arnaldo Carvalho de Melo <acme@mandriva.com>