Check the exit code from fsck_ffs instead of relying on MODIFIED being in the output

^/head@r323923 changed when MODIFIED is printed at exit. It's better to follow the
documented way of determining whether or not a filesystem is clean per fsck_ffs, i.e.,
ensure that the exit code is either 0 or 7.

The pass/fail determination is brittle prior to this commit, and ^/head@r323923 made
the issue apparent -- thus this needs to be fixed independent of ^/head@r323923.

PR: 222780
MFC after: 1 week
MFC with: r323923
Reported by: Jenkins