[CCID3]: Wrap computation of RFC3390-initial rate into separate function

[CCID3]: Wrap computation of RFC3390-initial rate into separate function

The CCID 3 and TFRC specs (RFC 4342, RFC 3448, draft-3448bis) make frequent
reference to the computation of the RFC-3390 initial sending rate:

  1. Initial sending rate when RTT is known (RFC 4342, p. 6)
  2. Response to Idle/Application-Limited periods (RFC 4342, 5.1)

This warrants putting the code into its own function, for later code reuse.

Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
Signed-off-by: Ian McDonald <ian.mcdonald@jandi.co.nz>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>