Since all attribute values start at 8-byte aligned boundary, we would
previously incorrectly calculate dn_bonuslen if any attribute but the
last had a variable-length value with length not multiple of 8.
Reported by: Nicolas Rachinsky <fbsd-mas-0@ml.turing-complete.org>
Tested by: Nicolas Rachinsky <fbsd-mas-0@ml.turing-complete.org>
Reviewed by: Matthew Ahrens <mahrens@delphix.com> (for upstream)
MFC after: 2 weeks