rbd: switch rbd_obj_method_sync() to ceph_osdc_call()

As explained in the previous commit, rbd_obj_request machinery (and
rbd_osd_req_create() in particular) shouldn't be used for working with
metadata objects.

Switch to the recently added ceph_osdc_call().  It assumes single pages
for outbound and inbound buffers, but that's OK - none of the callers
need more than that.  These pages need to be allocated (messenger is in
dire need of proper iterator interface!), but we are swapping for
pages[] and pagelist allocations in the existing code.

Kill class_name argument - all rbd methods are under "rbd".

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Reviewed-by: Jason Dillaman <dillaman@redhat.com>