Fix dirty check in dmu_offset_next()

The correct way to determine if a dnode is dirty is to check
if any of the dn->dn_dirty_link's are active. Relying solely
on the dn->dn_dirtyctx can result in the dnode being mistakenly
reported as clean.

Reviewed-by: Chunwei Chen <tuxoko@gmail.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #3125
Closes #6867