mds: Make min/max UID configurable for who is allowed to create a snapsh...

...ot

This way administrators can configure which UIDs are allowed to create
snapshots on CephFS.

In certain situations it might be desirable that regular users are not allowed
to create snapshots. For example on shared hosting environments.

Fixes: #9029
let's make this _mksnap_ or (better?) add the same check to rmsnap too?