cephfs: make is_mounted check less strict

cephfs: make is_mounted check less strict

checking '$server:$subdir' is too strict to work in all cirumcstances,
e.g. adding/removing a monitor would mean that it is not the same
anymore, same if one is adding/removing the ports from the config

check only if the subdir is the same and if it is a cephfs
this way, it still returns true if someone changes the config

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>