libceph: add CREATE osd operation support

Add CEPH_OSD_OP_CREATE support.  Also change libceph to not treat
CEPH_OSD_OP_DELETE as an extent op and add an assert to that end.

Signed-off-by: Yan, Zheng <zyan@redhat.com>
Reviewed-by: Ilya Dryomov <idryomov@redhat.com>