IO: Ensure that socket rcvbuf is large enough

IO: Ensure that socket rcvbuf is large enough

The socket structure has the field rbsize (receive buffer size), which
controls the size of the userspace receive buffer. There is also kernel
receive buffer, which in some cases may be smaller (e.g. on FreeBSD it
is by default ~8k). The patch ensures that the kernel receive buffer is
as large as the userspace receive buffer.