Fix checksumflags assignment in cksummer

drr_checksumflags was incorrectly set to drr_checksumtype.

Reviewed-by: Matt Ahrens <mahrens@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Signed-off-by: Tim Crawford <tcrawford@datto.com>
Closes #5830