mld6: Properly initialize MLD packet options

After commit 530c2c30b0c7 we need to set flags to ensure that hop-by-hop
and hop limit options are included.

Approved by: re (cperciva)
PR: 290407
Reviewed by: zlei, markj
MFC after: 3 days
Fixes: 530c2c30b0c7 ("ip6_output: Reduce cache misses on pktopts")

(cherry picked from commit 5568fba657c260286b9f1f1e837ab83ef6ce572d)
(cherry picked from commit 5c4144fe1f41b16368a3785b7edd6d2d70c24d1b)