UFS: panic rather than forcibly unmount the root fs

[chs]

Summary

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
Sponsored-by: Netflix

[karels]

Is there always leading context, like a failure messages? OtherwiseLGTM, but I'll let Kirk weigh in.

[imp]

There should always be context, but I'm hunting a 'weird' case in the disk error/timeout recovery code that might cause this w/o a message.
Usually there are very verbose error messages, though, that indicate why this was the case.

[mckusick]

This looks like the correct solution to me. We are at the point where we decide we need to do a forcible unmount and realize that we are about to do it on the root filesystem.