Initialize dn_next_type[] in the dnode constructor

It seems nothing ensures that this array is zeroed when a dnode is
freshly allocated, so in principle it retains the values from the
previous allocation. In practice it seems to be the case that the
fields should end up zeroed, but we can zero the field anyway for
consistency.

This was found using KMSAN.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Mark Johnston <markj@FreeBSD.org>
Closes #12383