Remove code duplication by moving code responsible for partial block
zeroing to a separate function: dnode_partial_zero().
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ryan Moeller <ryan@ixsystems.com>
Signed-off-by: Pawel Jakub Dawidek <pawel@dawidek.net>
Closes#12627