ssh_filter_btrbk: mbuffer validation too strict

I am using btrbk 0.32.5 on a server and 0.27.1 on a client and noticed that the validation of mbuffer on the server end is too strict.

Particularly, ssh_filter_btrbk enforces a verbosity level of mbuffer of 1, however, the client uses mbuffer without increased verbosity level. Thus, the client was rejected.

By making the parameter `-v 1` in `mbuffer_match` in [ssh_filter_btrbk.sh](https://github.com/digint/btrbk/blob/18ddc659790de489967c2c82213fb5fba7955d40/ssh_filter_btrbk.sh#L91) optional, the command is accepted:
```
mbuffer_match="mbuffer( -v 1)? -q( -s [0-9]+[kmgKMG]?)?( -m [0-9]+[kmgKMG]?)?( -[rR] [0-9]+[kmgtKMGT]?)?"
```