UFS: panic rather than forcibly unmount the root fs

If the root fs is forcibly unmounted then basically every process
will die with a SEGV as soon as it tries to run again because libc.so
is gone, which leaves the system basically hung. It seems better
to just panic instead, so let's do that.

Requested-by: karels
Reviewed-by: imp, mckusick, karels
Sponsored-by: Netflix
Differential Revision: https://reviews.freebsd.org/D41387