libceph: get rid of read helpers

Now that read_partial_message_pages() and read_partial_message_bio()
are literally identical functions we can factor them out.  They're
pretty simple as well, so just move their relevant content into
read_partial_msg_data().

This is and previous patches together resolve:
    http://tracker.ceph.com/issues/4428



Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>