libceph: let osd ops determine request data length

The length of outgoing data in an osd request is dependent on the
osd ops that are embedded in that request.  Each op is encoded into
a request message using osd_req_encode_op(), so that should be used
to determine the amount of outgoing data implied by the op as it
is encoded.

Have osd_req_encode_op() return the number of bytes of outgoing data
implied by the op being encoded, and accumulate and use that in
ceph_osdc_build_request().

As a result, ceph_osdc_build_request() no longer requires its "len"
parameter, so get rid of it.

Using the sum of the op lengths rather than the length provided is
a valid change because:
    - The only callers of osd ceph_osdc_build_request() are
      rbd and the osd client (in ceph_osdc_new_request() on
      behalf of the file system).
    - When rbd calls it, the length provided is only non-zero for
      write requests, and in that case the single op has the
      same length value as what was passed here.
    - When called from ceph_osdc_new_request(), (it's not all that
      easy to see, but) the length passed is also always the same
      as the extent length encoded in its (single) write op if
      present.

This resolves:
    http://tracker.ceph.com/issues/4406

Signed-off-by: Alex Elder <elder@inktank.com>
libceph: have osd requests support pagelist data

Add support for recording a ceph pagelist as data associated with an
osd request.

Signed-off-by: Alex Elder <elder@inktank.com>
libceph: kill osd request r_trail

The osd trail is a pagelist, used only for a CALL osd operation
to hold the class and method names, along with any input data for
the call.

It is only currently used by the rbd client, and when it's used it
is the only bit of outbound data in the osd request.  Since we
already support (non-trail) pagelist data in a message, we can
just save this outbound CALL data in the "normal" pagelist rather
than the trail, and get rid of the trail entirely.

The existing pagelist support depends on the pagelist being
dynamically allocated, and ownership of it is passed to the
messenger once it's been attached to a message.  (That is to say,
the messenger releases and frees the pagelist when it's done with
it).  That means we need to dynamically allocate the pagelist also.

Note that we simply assert that the allocation of a pagelist
structure succeeds.  Appending to a pagelist might require a dynamic
allocation, so we're already assuming we won't run into trouble
doing so (we're just ignore any failures--and that should be fixed
at some point).

This resolves:
    http://tracker.ceph.com/issues/4407

Signed-off-by: Alex Elder <elder@inktank.com>
libceph: kill message trail

The wart that is the ceph message trail can now be removed, because
its only user was the osd client, and the previous patch made that
no longer hte case.

The result allows write_partial_msg_pages() to be simplified
considerably.

Signed-off-by: Alex Elder <elder@inktank.com>
libceph: more cleanup of write_partial_msg_pages()

Basically all cases in write_partial_msg_pages() use the cursor, and
as a result we can simplify that function quite a bit.

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