In order for clean SACK LR after the RTO, we don't want to exit recovery, and we also need to clear the number of retransmitted sack bytes, for the cwnd calculation to work. Without these, the stack would oscillate between SACK and non-SACK retransmissions (snd_una), resulting in many duplicate sent packets.

There are two incidental edits here (missed one magic number and replaced with the proper enum, and a whitespace change).

Note that this approach is a bit brittle in my testing (with a pathological loss pattern of 3 lost - 1 delivered - 3 lost ..., as after the RTO, every packet is clocked out by a preceeding ACK (cwnd remains at 1, since I'm not resetting any of the prr variables, which could allow some cwnd growth during loss recovery).