mon: check cap before executing tell command

mon: check cap before executing tell command

tell command is sent from a client from network, we need to authorize
the cap of client before executing the command sent by it.

in this change, the check for `session->caps.is_allow_all()` is removed,
because

- simpler this way
- `session->caps.is_capable()` also check it, and this code path is
  not a critical path, so no need for trading the simplicity for
  performance here.

Signed-off-by: Kefu Chai <kchai@redhat.com>