Restore dirty dnode detection logic

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
(cherry picked from commit d7e640cf95f72deeca501d34afed59a0bc9d7940)

Approved by:	so
Security:	FreeBSD-EN-22:13.zfs