zdb -k does not work on Linux when used with -e

This minor bug was introduced with the port of the feature from
OpenZFS to ZoL. This patch fixes the issue that was caused by
a minor re-ordering from the original code.

Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Tim Chase <tim@chase2k.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Serapheim Dimitropoulos <serapheim@delphix.com>
Closes #8001