rbd: add missing switch arguments for recognition by get_command_spec()

rbd: add missing switch arguments for recognition by get_command_spec()

Currently this

  $ rbd --all children img

doesn't work, while this

  $ rbd children --all img

or this

  $ rbd children img --all

does.  The issue is that -a/--all isn't on the list of known switch
arguments.  The "rbd children" example may seem contrived but for more
complicated commands such as "rbd device map" mixing switches and
positional arguments occurs naturally:

  $ rbd device --device-type nbd --options try-netlink --show-cookie map img

Fixes: https://tracker.ceph.com/issues/53935
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>