mgr/smb: fix condition for smb earmark when cluster_id doesn't match

mgr/smb: fix condition for smb earmark when cluster_id doesn't match

This commit resolves an issue where accessing `earmark.split('.')[2]` would cause a
"list index out of range" error when the earmark is set to just "smb" without additional scopes.
The fix introduces a parsing function to safely handle earmarks, ensuring proper behavior
even when no cluster ID or additional scopes are present.

Fixes: https://tracker.ceph.com/issues/68342
Signed-off-by: Avan Thakkar <athakkar@redhat.com>