Fix fast path for uni directional transfers
pure ACK case:

drag snd_wl2 along so only newer
ACKs can update the window size.
also avoids the state where snd_wl2
is eventually larger than th_ack and thus
blocking the window update mechanism and
the connection gets stuck for a loooong
time in the zero sized send window state.

see PR/kern 55567

ok thorpej@, also found in FreeBSD

(kardel)