Migrate net.tcp.default_init_rwnd to net.tcp_def_init_rwnd

Currently, tcp receive window size is read from
net.tcp.default_init_rwnd then set to net.tcp_def_init_rwnd. It
should not use seperate property to read/write the value, it only
needs one of property basically. So migrate
net.tcp.default_init_rwnd to net.tcp_def_init_rwnd which has
formal API.

Bug: 182538166
Test: Manually check that net.tcp_def_init_rwnd has default
      value and proc/sys/net/ipv4/tcp_default_inti_rwnd node
      is created with same value.
Change-Id: I6748485f99198b1200c67d6595b659aac7d7e1e0