network - Restart the callout timer when setting TCP_FASTKEEP

network - Restart the callout timer when setting TCP_FASTKEEP

* When TCP_FASTKEEP is enabled we must reload the callout with the shorter
  timeout.  If we allow the longer timeout to expire and the connection
  remains idle the tcp connection will be dropped when the longer timeout
  expires since it will have exceeded the maximum calculation.

* Fixes noisy but otherwise benign NFS messages on the console.

* Abstract the tcp activity function into tcp_timer_keep_activity() so
  we can call it from both usrreq and tcp_input.