Only print buffer cache debug message when a cache lookup has been done.
When running `fsck_ffs -d` on a gjournal'ed filesystem, it's possible
that totalreads is greater than zero when no cache lookup has been
done - causing a divide by zero. This commit fixes the following error:
Floating point exception (core dumped)
Reviewed by: mckusick
Differential Revision: https://reviews.freebsd.org/D30370