Revert "Remove wrong ASSERT in annotate_ecksum"

This reverts commit 093911f1945b5c164a45bb077103283dafdcae0c.

Reviewed-by: Chunwei Chen <tuxoko@gmail.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Closes #7079