stable/12 mount nocover errors from linux rc script

[Edward Tomasz Napierała]

On 0828T1937, Sean C. Farley wrote:
> I recently updated to 12-STABLE r364926 and noticed errors from 
> /etc/rc.d/linux.  When mounting devfs and tmpfs, errors are reported 
> about the nocover option.  It looks like cover/nocover was introduced in 
> HEAD with r352614 but has not made it to 12-STABLE.

Thanks, and sorry for the breakage; I've committed a temporary workaround.