crimson/net: throw read_eof if short read

crimson/net: throw read_eof if short read

as per the implementation and the document of input_stream::read_exactly():

/// stream and returns them. If the end of stream is reached before n
/// bytes were read, fewer than n bytes will be returned - so despite
/// the method's name, the caller must not assume the returned buffer
/// will always contain exactly n bytes.

Signed-off-by: Kefu Chai <kchai@redhat.com>