libceph: initialize data fields on last msg put

When the last reference to a ceph message is dropped,
ceph_msg_last_put() is called to clean things up.  For "normal"
messages (allocated via ceph_msg_new() rather than being allocated
from a memory pool) it's sufficient to just release resources.  But
for a mempool-allocated message we actually have to re-initialize
the data fields in the message back to initial state so they're
ready to go in the event the message gets reused.

Some of this was already done; this fleshes it out so it's done
more completely.

This resolves:
    http://tracker.ceph.com/issues/4540

Signed-off-by: Alex Elder <elder@inktank.com>