ddb: do not assume that ffs is mounted with softdep

Avoid a panic when debugging with "show ffs" in ddb.

Reviewed By: kib, markj, mckusick
MFC after: 1 week
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D31622

(cherry picked from commit 3b29c8b4bdb0e396d7bb5d0760f11d7fa4208c84)