ofpbuf: Make ofpbufs initialized with ofpbuf_use_stack() not expandable.

ofpbuf: Make ofpbufs initialized with ofpbuf_use_stack() not expandable.

My original intent for ofpbufs initialized with ofpbuf_use_stack() was that
the caller was providing enough space on the stack for the common case,
with dynamic allocation as a fallback.  But in practice, none of the
clients actually do this.  Instead, all of them actually know that the
stack-allocated buffer is big enough and, since they don't want to bother
with having to call ofpbuf_delete(), they instead assert that the buffer
wasn't reallocated.

Since this is a bit of a pain, this commit changes the semantics of
ofpbuf_use_stack() to be that the stack-allocated buffer cannot be
reallocated at all.  This is more convenient for the existing clients.