Hide the "unmount of /dev failed (BUSY)" warning at shutdown or reboot,
introduced with r293742, just like it was hidden before that commit.

This is a direct commit to 10-STABLE; this special case is not needed
in 11-CURRENT, because devfs supports forced unmounts there. The forced
unmount could be MFC-ed, but there are some LORs at shutdown, and I have
a weird feelings about it.

Sponsored by: The FreeBSD Foundation