fix bug introduced in r297521, set canmount=on doesn't mount filesystem

There are two cases where changing canmount should result in an action:

• canmount is set to off for a mounted filesystem
• canmount is set to on for an unmounted filesystem

Before r297521 we could unmount and re-mount a filesystem when that was
not necessary, but after r297521 we only handled the first of the above
cases.

MFC after: 5 days