network - Fix a sleep/wakeup race in NFS (server), implement faster keepalives

network - Fix a sleep/wakeup race in NFS (server), implement faster keepalives

* Fix a sleep/wakeup MP race in the NFS server where the server can receive
  data but fails to wakeup one of its nfsds.

* Set a faster keepalive timeout for NFS client and server.  Currently
  75 seconds (instead of 2 hours).  This solves a problem when connectivity
  is lost with the server for long enough that the server kills the
  connection, or if the server fails while processing a nfs request
  from the client.

  If the client is stuck waiting for the reply it can wind up waiting
  for the entire keepalive period before recovering, even if connectivity
  with the server is re-established.  This way the client will proactively
  determine that the connection is dead within 75 seconds and re-establish
  it.

* The value of the timeout is taken from net.inet.tcp.keepintvl
  (instead of net.inet.tcp.keepalive), which defaults to 75 seconds
  at the moment.