Alexandre, please see my last comment, that links to a test kernel PPA with EXACTLY that upstream commit, 751eb6b6042a596b0080967c1a529a9fe98dac1d ("ipv6: addrconf: fix dev refcont leak when DAD failed")
If you can reproduce this, please test with the PPA and report if it fixes the problem.
Alexandre, please see my last comment, that links to a test kernel PPA with EXACTLY that upstream commit, 751eb6b6042a596 b0080967c1a529a 9fe98dac1d ("ipv6: addrconf: fix dev refcont leak when DAD failed")
If you can reproduce this, please test with the PPA and report if it fixes the problem.