Restore dirty dnode detection logic

In addition to flushing memory mapped regions when checking holes,
commit de198f2d95 modified the dirty dnode detection logic to check
the dn->dn_dirty_records instead of the dn->dn_dirty_link. Relying
on the dirty record has not be reliable, switch back to the previous
method.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #11900
Closes #12745