boottrace: Don't say we've unmounted if we haven't.

MFC after: 1 week
Sponsored by: Klara, Inc.
Sponsored by: NetApp, Inc.
Reviewed by: imp, markj
Differential Revision: https://reviews.freebsd.org/D48515

(cherry picked from commit 4f4b2b7a4b6c5642320479a765f850c394766276)