libceph: Simplify messenger_v1

Simplify the messenger_v1 I/O transport.  The data type is no longer
necessary and we can just use the iterator.  On top of that, the msg_data
iterator routines can be simplified a bit and the transmission loop now
only needs the sendmsg-from-iterator branch.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Ilya Dryomov <idryomov@gmail.com>
cc: Xiubo Li <xiubli@redhat.com>
cc: Jeff Layton <jlayton@kernel.org>
cc: ceph-devel@vger.kernel.org
cc: linux-fsdevel@vger.kernel.org