The value returned in the "snap_name_len" argument to
rbd_add_parse_args() is never actually used, so get rid of it.

The snap_name_len recorded in rbd_dev_v2_snap_name() is not
useful either, so get rid of that too.
Signed-off-by: NAlex Elder <elder@inktank.com>
Reviewed-by: NJosh Durgin <josh.durgin@inktank.com>