Report holes when there are only metadata changes

Update the dirty check in dmu_offset_next() such that dnode's
are only considered dirty for the purpose or reporting holes
when there are pending data blocks or frees to be synced. This
ensures that when there are only metadata updates to be synced
(atime) that holes are reported.

Reviewed-by: Debabrata Banerjee <dbanerje@akamai.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #6958
Closes #8505