When evicting from the ARC, we manipulate some refcount_t's, e.g. arcs_size.
When using zdb to examine a large amount of data (e.g. zdb -bb on a large pool
with small blocks), the ARC may have a large number of entries. If reference
tracking is enabled, there will be ~1 reference for each block in the ARC. When
evicting, we decrement the refcount and have to search all the references to
find the one that we are removing, which is very slow.
Since zdb is typically used to find problems with the on-disk format, and not
with the code it is running, we should disable reference tracking in zdb.