Revert "Report holes when there are only metadata changes"

This reverts commit ec4f9b8f30 which introduced a narrow race which
can lead to lseek(, SEEK_DATA) incorrectly returning ENXIO. Resolve
the issue by revering this change to restore the previous behavior
which depends solely on checking the dirty list.

Reviewed-by: Olaf Faaland <faaland1@llnl.gov>
Reviewed-by: Igor Kozhukhov <igor@dilos.org>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #8816
Closes #8834