libceph, rbd, ceph: move ceph_osdc_alloc_messages() calls

The current requirement is that ceph_osdc_alloc_messages() should be
called after oid and oloc are known.  In preparation for preallocating
message data items, move ceph_osdc_alloc_messages() further down, so
that it is called when OSD op codes are known.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>