DDT: Make children writes inherit allocator

Even though unlike gang children it is not so critical for dedup
children to inherit parent's allocator, there is still no reason
for them to have allocation policy different from normal writes.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Rob Norris <robn@despairlabs.com>
Signed-off-by: Alexander Motin <alexander.motin@TrueNAS.com>
Closes #17961