msg: prefetch data when doing recv()

Since that when we're reading the header we do lot's of small IOs, we
can really benefit from doing bigger reads.
Set a limit on the buffer size, so that it should mainly affect header
reads and not the actual data transfer of large IOs.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
i think even 128kb is plenty for a super high-end fabric.  But i would be happy to just drop this.. not sure we need to worry so much about users putting stupid values in here. The behavior will still be correct, just waste memory.