MFZoL: Fix issue with scanning dedup blocks as scan ends

MFZoL: Fix issue with scanning dedup blocks as scan ends

This patch fixes an issue discovered by ztest where
dsl_scan_ddt_entry() could add I/Os to the dsl scan queues
between when the scan had finished all required work and
when the scan was marked as complete. This caused the scan
to spin indefinitely without ending.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Serapheim Dimitropoulos <serapheim.dimitro@delphix.com>
Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Signed-off-by: Tom Caputi <tcaputi@datto.com>
Closes #8010
zfsonlinux/zfs@5e0bd0ae056e26de36dee3c199c6fcff8f14ee15

MFC after:	1 week