The only user of the ceph messenger that doesn't define an alloc_msg
method is the mds client. Define one, such that it works just like
it did before, and simplify ceph_con_in_msg_alloc() by assuming the
alloc_msg method is always present.
This and the next patch resolve:
http://tracker.ceph.com/issues/4322
Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Greg Farnum <greg@inktank.com>