change default: by default, use socket buffer auto tuning

  /* I don't think that a tcp send buffer of more than 10M is usefull */

#define DRBD_SNDBUF_SIZE_MIN  0

#define DRBD_SNDBUF_SIZE_MAX  (10<<20)

#define DRBD_SNDBUF_SIZE_DEF  (2*65535)

#define DRBD_SNDBUF_SIZE_DEF  0

#define DRBD_RCVBUF_SIZE_MIN  0

#define DRBD_RCVBUF_SIZE_MAX  (10<<20)

#define DRBD_RCVBUF_SIZE_DEF  (2*65535)

#define DRBD_RCVBUF_SIZE_DEF  0

  /* @4k PageSize -> 128kB - 512MB */

#define DRBD_MAX_BUFFERS_MIN  32