dont alloc buf and make iovec in read-request parsing, insist on buffer in read-request encoding

[bdodge]

• In processing read request pdu's I had allocated a reply data buffer and iovec for it so that when a reply is made, there is a buffer in the req already. This isn't really needed since the server application might want to own this buffer instead and encoding the reply makes its own iovec with the buffer passed in so the previous iovec isn't used, so I deleted both the alloc and iovec creation.
• In generating a read request, I added an error if no buffer for the reply data is passed in. There was only one call to this function in libsmb2.c which does pass in a buffer.